About speed, I really do belive that SDL is major slowdown for 68k amigas, but we will see is it true, when native scummVM is relased.
Quite a few RTG 060 users have said the AGA version of ScummVM is much faster on their RTG boxes than the SDL based RTG release so yes, I hope when I do a 'native' RTG version we'll see the speed increase a bit (it should of course run faster than the AGA version at the same resolution/color depth).
The AGA version may get a chance to catch up again with the RTG version if I can get direct-chunky working (eg Graffiti) and skip the C2P step
I don't think the 68k SDL is a curse though, it was very helpful to me when I did my intial port (the 68k SDL source code was made open source). Most of the performance issues are probably down to the fact that it's no longer maintained.