As for the ReAction vs MUI war, I tend to think there aren't any good independent GUI toolkits out there. That, and MUI is slow as a dog under WinUAE on my Athlon 2600+! I haven't used ReAction. I tend to limit my WinUAE sessions to old games these days.
What's wrong with yourJIT-WinUAEsetup?
MUI is fine in my AthlonXP/nForce2/JIT-WinUAE box and it’s also fine in my older Athlon@1.3Ghz/VIA KT/JIT-WinUAE box.