I've wondered about something similar.

The original roadster was based on the elise (similar to exige), and it was heavier.

I think the highest performance models now have the largest batteries mostly due to marketing. basically "If you want the performance, pay for more battery than you need".

Why not have a 35kwh car, a supercapacitor as a buffer and have it weigh significantly less for better performance? (at the expense of range)