I haven't experienced any lag at all, but I have seen the occasional screen tear. I've played through on Sith Lord difficulty and I'm at the end of the final level, but annoyingly, the final confrontation is far too difficult on this setting. Good fun, but there are some really frustrating moments in the game (the end of the penultimate level for example).
If you have your Xbox standing upright, turn it onto it's side and have it on a solid surface, e.g., a wooden platform so it's not sucking in dust from your carpet. I noticed also that when i turned my xbox onto it's side, the noise reduced dramatically as well.
Apparently aswell, if you have your Xbox vertical, it is more likely to damage your disc, as is what happened with mine untill i lay it on it's side, now my discs don't get the strange circular scratches.