>Do you really think that OS4 on NatAmi would allow more banging than OS4 does on a PowerUP Amiga?

No, I think it would offer a similar level of capability on better hardware. Even the PPC could be improved. I've been pleasantly surprised to see Powerup OS4 machines running 68K games better (and at higher resolutions) then on 68 legacy hardware.
A combination like this could run 68K software better then legacy hardware and PPC software better then Poerup hardware.