MyGP2X have interviewed Franxis the coder of Mame of the GP2X, heres an excerpt:
How well has your emulator performed in comparison to the real thing?
In GP32 i think it is impossible MAME to run perfectly. Then i spent a lot of time optimizing and doing a lot of hacks to make MAME in a acceptable performance.
GP2X console has more power and i have seen a lot games running ok without such hacks and at 200 MHz. Then i think the difference betweek GP32 to GP2X is more than the 166 MHz (GP32) to 200 MHz (GP2X).
Read the full interview here --> http://www.mygp2x.com/blog_comment.asp?bi=246