My understanding of using the 8x bandwidth you should really only see a minimal drop in performance in highly demanding graphics games. In most circumstances fully saturating the 8x bandwidth just doesn't happen.
Then that was a few years ago before games like Crysis, Batman Arkyum Asylum, or any newer DX10 or 11 games.
I still wouldn't expect you should see any major loss of performance in most games/programs.
Fabe