I will make only this one attempt at beating a dead horse, but I still think supercapacitors have good characteristics as a performance booster pack. The idea is a special high performance model, priced several 10s of thousands higher. Rather than compromise using special batteries with a higher power density but lower energy density, which compromises range, there would be division of labor between components specialized for energy density (batteries) and power density (capacitors), retaining long range and adding high intermittent power without too much weight. Yes, the interplay of two power sources requires a lot of power electronics intelligence, but so does the car now, with two motors, alternators, a wide operating range, regen, etc. The combination would not only offer drag strip blasts, but also vastly stronger regenerative braking, probably up to 1 g (which would need to be integrated with the pedal, which it is not now). Yes, energy would need to be moved back and forth between the capacitors and battery pack intelligently, but at a slower rate (power level) reducing thermal load on the pack and its long-run degradation.
The objective would not just be occasional acceleration runs, but track day performance with the capacitors supporting braking into, and acceleration out of corners. A further advantage of capacitors here is that they are good for an extremely large number of charge-discharge cycles relative to batteries. Using batteries to store and release energy in each corner might work for a KERS system lasting a race or two, but not for the life of the car. With this system, the 'Ring may become possible without reversion to limp-home mode, although enhanced motor cooling would probably be required.
This idea may have been born from a large martini last night (see earlier post) but it's really growing on me.
Editing: I originally said capacitors in the frunk but on reflection, that would compromise the front crumple zone and crash safety. They should probably go under the rear floor where the jump seats would fold up.