I do believe that turbines run most efficiently when they are crazy hot, but maybe they can make a small enough one that can dissipate the heat quickly. This page has a little more info on the jag, specifically that the turbines produce 188hp!
Jaguar Unveils Supercar Hybrid Concept - MSN Autos
I'm sure it needs 188hp of re-powering to keep up with the possibility of the driver doing repeated 0-60 times in the jaguar, but if any of us were driving, what would be the optimal average energy output?? I would think we could get by with 40hp or even 30hp being continuously produced. Maybe even less if they put it in a really aerodynamic car?