thats pretty good couchmonkey!
here is what you listed: Citadel, Star Fox, Stunt Race FX, Super Mario World 2: Yoshi's Island, and...oh crap, there was some bike racing game, but I can't remember the title now. All I remember is that it used sprites on 3-D backgrounds.
that makes 4....now there is Doom for snes that uses the fx chip as well....
and the bike racer you're thinking of was Dirt Trax Fx! when this game was previewed in
Nintendo Power back in the early 90s, I was watching this game like a hawk...
the graphics were simply amazing IHO, and the game looked like it would play like a dream...
so I waited and waited YEARS for this game, and after waiting so long I just about forgot about the game
until one day my friend picked in up in bargain bin at kbtoys...and I finally got to play it
it was just as I thought it might be....awesome in all aspects! - graphics, gameplay, multiplayer xtras, etc
so citadel, starfox, stunt race fx, super mario world 2:yoshis island, doom, and dirt trax fx!
if anyone knows any more games that used the fx/fx 2 chip...feel free to post it up