Topic on Talk:Final Fantasy XIII
http://forums.eu.square-enix.com/showthread.php?t=149223&page=5#post2096444
http://www.pcgamer.com/final-fantasy-xiii-and-xiii-2-port-analysis-durantes-verdict/
It's just a bug with game then. As I said, it doesn't make sense that limiting the framerate -thus actual expressed horsepower in moment X- is going to produce better result for frame X+1 (assuming software/drivers are not flawed of course)