The answer should be that supercharging degrades battery life but anecdotal evidence is pointing to the opposite. A clue can be found here around minute 6:
In which Dahn says that it's not how fast you charge it but how long it spends charging because more time at raised temperature increases parasitic reactions.
The thermal management will keep the battery from rising above x temperature. If you charge at home at 40 amps on a moderate to warm day, the AC will eventually kick in to cool the battery and keep it at x temperature. Say you charge for 4 hours on an 85 to get a 50% SOC increase. The battery has spent 4 hours at x temperature incurring degradation from parasitic reactions.
Now say you supercharge. The battery should get a lot hotter, right? Not necessarily. The thermal cooling will kick in at a much higher rate when needed to keep the battery at the same x temperature threshold. So now you've only spent 20 minutes for the same range and same battery temperature vs 4 hours at 40 amps.
He clearly states that it's not the rate at which you charge but how long the battery stays warm from charging. So if you can charge fast and use energy to keep the battery cool, it will last longer than if you spend more hours charging the battery at the same temperature regardless of the charge rate.
Note, I'm comparing a moderate charging speed in moderate to warm weather to supercharging. Charging at 1.2KW from 110v will not result in any consequential battery temperature increase so that would be the best way to treat the battery even though it takes 1.5 days to get the same 50% increase in SOC.
I guess what I just said is that I think charging at moderate speeds causes the most degradation while supercharging or super slow charging causes the least.
So what I want to say is, no don't supercharge because you'll ruin your battery but I suspect it's not even remotely true.
So instead I'm going to say, be responsible and don't locally charge especially if there's a line of people who are passing through on long distance trips. I can't STAND local chargers because here in the Bay Area, most of the folks charging are doing so to save money from charging at home.
Tesla's change in unlimited couldn't come fast enough for new cars and idling policy for all cars. Hopefully they will tighten down the free for life to actually enforce the "for long distance travel" disclaimer that they put on their site back in early 2015. I suspect those that bought befor ethen won't have to adhere to a "long distance only" charging policy since Tesla's didn't say "for long distance only" prior to that.
