I remember Gnostic's UAE2x, and the subsequent hilarity on the forums when UAE4ALL was released.
Basically, UAE4ALL handed UAE2x's arse to it on a plate in terms of speed and compatibility - it really was exceptionally fast, running the vast majority of games at full speed on a GP2X clocked at 200mhz, where previously UAE2x could only manage few games when overclocked. Reports are that UAE4ALL has better compatibility, but seeing as very few people are testing UAE2x there's not much hard evidence to go on.
Gnostic is noted as one of the developers of UAE4ALL, along with Notaz and Critical. I don't know if Gnostic will be going back to UAE2x now that UAE4ALL is so much more matured, or if he will have any input with the Pandora port of UAE (whichever source is chosen).