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.
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That would be cool, whould it extend the battery/stelth usage time?