mikochu,
It's tough to say authoritatively what you mean by "performance", but I'll take a stab anyway.
I'd say as long as your TV handles game FPS ok, just set it up for 1080 and be done with it.
I don't really see what all the fuss is about. I use 1080 because it improves the pixel resolution of the virtual driving arena. And at least for me, I do not really "need" 60 FPS to gain what I purchased GT6 for. The screen resolution more than pays for any minor reduction in framerate.
Based on a wild guess that like the vast majority of players, (and people in general), the benefits in terms of resolution performance will far outweigh any reduction in FPS performance, and you'll see the best payback by using 1080 and leaving 720 to those who are still tied to 720-only CRT TVs or have panty-twists because it's not really "real" or the FPS isn't 60. It's really not that big a deal.