Actually lithium has a much better C-rate than just several years ago. Existing LG-chem LiIon cells can kick out 3 kW per kg and A123's new cells now being manufactured for power tools can charge / discharge 90% of capacity in just 5 minutes (ie at least 10C).

But I still think you're right - ultracaps paired with high energy LiIon could be a very good combination for longevity.

http://mysite.wanadoo-members.co.uk/ecotech/ecot.htm