I agree that ReGBA (from gpSP) is not that great, but that's because Exophase has not had the opportunity and drive to finish up some things that are needed for full compatibility (and the shortcuts it takes for performance during emulation may hinder that compatibility). It's no VBA-M, that's for sure. It's a fully custom core.
As for PocketSNES, it's issued from Snes9x 1.43-dev, not a custom core
, with optimisations and fixes on top for things like audio sync, pseudo-hi-res (Secret of Mana, Seiken Densetsu 3, Rudora no Hihou menu text etc.), imports of code for SNES special chip support, and so on.
Star Fox runs at 40 FPS in-game on this version of PocketSNES on the GCW Zero. Kirby games run close to full-speed. Mario RPG and Yoshi's Island are very location-dependent in-game, but vary from 40 to 60 FPS. Some esoteric games will be supported in 1.53 but not 1.43, but I expect that number to be pretty small.
Using Snes9x 1.53 or Snes9x Git on the GCW Zero yields 50-60 FPS in Super Mario World, which is a very simple game to emulate, and Yoshi's Island runs at 12 FPS. Please see /GCW Zero/Development/Snes9x 1.53 and Git: slow, as I expected
for more on that.