Well, if you want to play in simple 2D then 68000/68020 are limited by the graphics' chipset. If you want to do anything more, then even 68030 is limited. Games like X-Treme racing that were extremely good, could run properly (1:1 pixels) only on 68030.
The PCs that you mention are another story. Amiga did not have excess power after a point. This point is called 3D games. Amiga was not capable of producing 3D games at 25FPS. Even Doom, that is not actually a 3D game runs at low fps on the Amiga and is unplayable on 68020s. This is actually my main point of this article? How much a 68030/50 - 040/25 that were very common expansions among the Amiga users would benefit from games written in ASM. I am not talking about 2D games, Amiga was good in that and we all know it.