FPS aside, they are purists who will argue that MAME is not 100% perfect compared to a real machine. I myself can't tell the much difference with a majority of the games, but there are those who can. Of course I'm not refering to things MAME wouldn't be able to replicate exactly like the Q*Bert knocker, rank lights on Gorf, etc., but instead the minute things like slight change in color palette or pitch of a sound effect.
If you're hardcore like that, then I suppose nothing would please you other than a Jamma (or dedicated Neo-Geo MVS) cabinet.
However, since you mentioned specifically FPS - I found Neo-Geo to run fine even under my 1.8Ghz P4 (can't play MKs too well though). From what I can recall - nothing seems terribly "off" when compared - from memory mind you - to the real deal.
Most people don't seem to mind those tiny differences (and I still can't convince people that vinyl sounds better than CDs). Even still, I wouldn't mind owning a Jamma cabinet and a few boards, just for authenticity's sake....