Also Maxwell's ultracapacitors are spectacular, and could enable a lot of high end features and/or improve regenerative braking across the board. On the high performance models, the motors already tend to move more power than warm summer tires in the dry can even transmit; I'm not sure what the ratio of regen to mechanical/heat braking currently is, but it seems like they could do most non-emergency braking through the motors. Not sure if the batteries are the limiting factor on that right now though.

Once the battery is warm most braking is already regenerative unless you are driving quite aggressively and need to reduce speed very rapidly. At least that's how it seems to be in my 2015 S 70D.

So ultra capacitors would have the most impact where people have short commutes in cold climates. Would be handy for me and might make a significant difference in the winter as the nearest thing I do to commuting is a 22 km journey into town and the same coming back after the car has been sitting idle for hours. It takes between 5 and 10 km for the battery to be warm enough for regen to be effective.

But after that I can get 50 kW of regen which is plenty enough to slow the car in normal circumstances.

I'd say Tesla bears tend to think that the focus to go into battery production is mainly motivated by the ongoing deterioration of their relationship with Panasonic.