There is a reason why this question is so frequent, it is because the EV industry, powerful all electric car batteries (they are generally more powerful than hybrid vehicle batteries) and specially the concept of fast charging at commercial scale were still a recent phenomena that there hasn't been enough time to do conclusive testing and arrive at conclusive answers.
However, this is no longer the case. Recently several research, tests and analysis with medium term data collection has been done, to such a degree that we now do have some data to answer this question. Yea, there could still be some nagging minor doubt here and there on exact numbers, but in general the question can be reasonably evaluated.
In older days (about 5 years ago or more), understanding was that fast charging may damage the battery a bit over long time but no one was sure by exactly how much by over 8-10 years of usage, since lack of data. Nissan and Tesla both now seems to be thinking that the long term battery degradation due to quick charging is not as bad as previously thought. Infact they now think that the total number of miles travelled is the main factor affecting battery life than the method of charging (L2 or L3). You can see this by comparing Nissan Leaf owners manual (UK) in 2011 with the one they issued in 2015
2011 LEAF owner's manual:
Use the normal charging or trickle charging methods to charge the Li-ion battery and minimize the use of public Fast Charge or Quick Charger.
Avoid exceeding 70-80% state of charge when using frequent (more than once per week) public Fast Charge or Quick Charging.
NISSAN recommends that quick charging not be performed more than once a day
2015 LEAF owner's manual:
NISSAN recommends using normal charging for usual charging of the vehicle. Use of quick charge should be minimized in order to help prolong Li-ion battery life.
Avoid sustained high battery temperatures (caused, for example, by exposure to very high ambient temperatures or extending highway driving with multiple quick charges).
Note how soft the wording in 2015 version of the manual (compared to strict warnings in 2011), also they are putting more emphasis against high temperatures instead. Nissan web site actually goes further and even recommends fast charging.
See here Note this section specifically.
Rapid Charging at electric car charging stations allows a LEAF to charge from 0% to 80% in approximately 30 minutes. Available across almost the entire UK motorway network, you can also find them in Nissan Dealerships, IKEA and Waitrose, plus various other locations.
Nissan recommend the use of the Ecotricity Electric Highway as it’s currently free (you will need to register first) and provides access to a wide range of Rapid Chargers to help you on those longer journeys. For example, using the Electric Highway on a journey from London to Bristol could cost you absolutely nothing!"
Word "recommends" is now used without restraint but at the same time they have managed to sneak in the "0% to 80%" just to make sure! This kind of summarises the present understandings and research stats.
Infact, even as early as 2012, they have got bit bolder. Speaker here is Mark Perry, Nissan North America's Director of Product Planning.
At first he uses strong assured phrases such as "Nissan is designed with fast charging in mind', "2-3 times of fast charging daily, no problem", "car is intelligent enough to protect it self" etc but when grilled, says there could be small percentage difference over 10 years, so the optimal is to charge up to 80% but still he maintains there is no significant damage in 100% charging (except 'small percentages over 10 year') and go for it if we really need it.
This is another section from the manual :
- Info on charging in LEAF Owner's Manual: "Overview" / "Efficient use of your vehicle" / "Li-ion battery life" section.
- Only charging to 80% in order to maximize battery life:
"Long life mode
NISSAN recommends charging the Li-ion battery using the long life mode to help maximize the Li-ion battery useful life. Long life mode can only be set using the charging timer function. The long life mode is set by changing the [% Charge] to [80% Charge (Improves Battery Longevity)] using the following procedure." 
Furthermore, the LEAF's warranty includes an exclusion regarding Li-Ion battery being charged full on a daily basis "despite the lithium-ion battery keeping a high state of charge level (98-100%)." 
- Allow the battery charge to be below at least 80% before charging.
- Avoid leaving vehicle for over 14 days where the battery charge state is zero or near zero.
- Allow the vehicle and Li-ion battery to cool down after use before charging.
- Only charging to 80% in order to maximize battery life:
- If vehicle will not be used for long period of time, NISSAN recommends charging with "long life mode" (charging only to 80%, see above) and to charge once every 3 months
Anyway, Nissan may be interested in its business and if the battery died after 8 years, the burden will be on users so we need some neutral party to come up with test data. Given that it is still 5 years since the launch of commercial level EVs, those are hard to come by.
I managed to find one such at following : http://www.nrel.gov/docs/fy15osti/63700.pdf (Let us know if you find more.) Here too they agree that the long term impact is not significant.
Another angle to look at the issue is to ask the question 'what happens at quick charging which may cause problems for the battery'. The answer is, high power is associated with high temperatures! High temperatures as we know are not good for batteries. Larger the power, higher the temperatures can go while charging. This is why no body even asking the question of damage due to slow L2 charging. (however the 'optimal' habit of not charging more than 80% applies to the L2 charger as well as mentioned earlier.)
Therefore, if we are really concerned, it would be prudent to minimize whatever damage and risk by using less power as possible. 40KW would be better than 60KW, 30KW would be better than 40, 20 would be bette than 30, and L@ charging would be safest of all etc but we can not wait long period to charge the battery if we are to use our cars in practical day to day usage. So at some point we will have to draw a line and find the correct balance between practical usage and ultra safety over long term.
Personally, I myself of the opinion that 20KW provides us that balance. It is fast enough for most of our requirements (I'm yet to find a case that it exceeds one hour for a charge), cheap enough to afford fair prices to EV owners, safe enough for batteries against high temperatures since it uses less power. However, I would not write off 30KW either.
Using beyond that (40KW or more), I think is a mistake for Sri Lanka. Here are the reasons :
1) Our grid provides only 60A max with 3 phase, beyond that you need a dedicated paid transformer.
2) If you go for such a transformer, then that huge cost will usually come to the end customers but why should EV owners pay for transformers ?
3) If you do not go for a transformer then you can draw only 60A, meaning your 40KW charger is actually under utilized. If thats the case, why should end customers pay more for power they are not even using ?
4) Risks associated with long term battery life. (however minor they can be)
To summarize then :
1) When possible, charge only up to 80% (whether it is L2 or L3)
2) If you really have to charge up to 100% to get the range, don't worry too much over it but at the same time, try not to do it too often either, just to be on the safe side.
3) let the battery cool down after the drive before charging. (since we know that the temperature is the enemy)
4) less power the better for the battery so whenever possible, use lesser KW if you are very concerned about long term battery life.
5) Reduce amounts of occurrences where the battery is discharged very low. If it goes too low, let it cool down a bit and charge. Dont keep the battery in Low of High charge states for longer durations.