A few months ago someone did extensive testing on the lag that MAME introduces to games (not based on hardware at all, entirely from the code of MAME). There are a lot of shmups that have an extra frame, or several extra frames of lag because MAME adds some sort of bugger. Someone found out that you could safely remove this buffer from several game drivers without a problem.
http://shmups.system11.org/viewtopic.php?f=1&t=30659dhex, I know you like to play a little DonPachi on MAME somtimes, so you might want to check this out. MAME adds an extra frame or two of lag, which is quite a bit more than you might think, so this will feel much more responsive.