What year and model exactly? If it's a later car, removing the smog equipment will yield a noticeable increase in power. Be sure to check local laws regarding smog equipment first though! Some people claim removing the center muffler will help give the engine a boost but I doubt you would notice more than a different (deeper) exhaust tone.
THe best thing you can do is have the engine properly gone through by a good MG tune-up guy. I have mine gone through once or twice a year just to make sure I haven't screwed up my settings too badly! /ubbthreads/images/graemlins/wink.gif