I think it is a myth that OS friendly 68K programs are executed faster on OS4 or MOS, because their emulation is more lightweight.
If you are not playing games, but running a "High-End" Amiga, you don't need cycle exact emulation of Custom Chips. Actually, apart from CIA, you don't need any Custom Chip emulation if you are using an RTG Screen and AHI Sound, and WinUAE is doing exactly that.
So what has to emulated is redicously cheap to emulate and costs a neglegtable amount of CPU Power. The execution of 68K programs under WinUAE has NO handbrake on, as people post here.