Yea, that is the problem with the hybrids. As far as I'm aware, they're all small petrol + electric units, so it's pretty good for low mileage start/stop town driving (electric motor only). For highway use, you're basically running a small petrol unit, plus carrying the weight of an unused electric motor and batteries. Frankly, most people would probably be better off with a small Diesel.
As for the 'vette - remember that at constant speed, mass is less of an issue (aerodynamics is more important). Also, the 'vette would probably be chugging along with low engine revs, but the hamster in the hybrid would be spinning that wheel real fast
.
The BBC's Top Gear program did an interesting, if not particularly useful, comparison - drive a hybrid as fast as possible round a track, and follow it with a BMW M3. The M3 returned better mileage.