I'm also working (still...) on isometric 3D game engine, my game will be strategy game, much like UFO (in looks) and Laser Squad (in playability, 2 players, anyone?)
I'm currently rewriting GFX routines to take better advantage of 24bit display...
Can't say much about speed yet (sure it's fast enough on 600MHz PPC :-) ), but I haven't optimized gfx routines much yet (And likely won't, before the game is otherwise more complete)
But I'll likely be making "Classic Amiga" version aswell, but it will likely require GFX card (unless I do some serious altering in GFX routines, and it sure won't look as good in 8bit)