Any performance mod will net more mpg if you can keep your foot out of it, straight up. Works even better if you swap in to a lighter car or lighten whatever car you have to begin with. The greater power can move around the smaller mass, easier.
As a counterpoint to the philosophical discussion that has ensued, check out the diesel Audi R10 prototype racer. It's a multiple time winner, in part due to it's greater efficiency; has to pit less often = fastar as a race car. They're debuting a hybrid version this year at 24-hr Les Mans, and Peugeot was developing a hybrid of their own until they pulled out of the season entirely.
I agree that hybrids are silly and pretty much a marketing gimmick in large part (why not just better engineer existing full gas or diesel engines, the r&d certainly can't be cheap for hybrid stuff, so there's money for it..?). But for the sake of discussion, there are places where hybrid makes sense and can do more than a pure combustion engine can, alone.
