I don't know about any "performance" differences. However I own an 07 STi and my cousin has an 08. His 08 has a ton more cargo space due to being a touch longer and a hatch. The 08 just seems much more refined, is that the word I am looking for? Its a smoother, quieter ride with the same great performance feel. I really don't like the body style, but the 08 just seems to be a much more 'finished' car than the 07's and prior. If I could get the ride comfort is his and the body of mine together it would be perfect
I can touch on performance in the fact that they don't seems to be too different from each other. The 08 seems like it might be a little more stable in hard cornering, but that's it as far as I could feel between the two.
Hope that helps you in some way.