don't know about games but the A500 was exploited quite a bit by demos, have you seen them?
Piru's right about WB of course. Going from 4 to 8 colors cuts the speed by roughly a third, going to 16 colors looses another third. Whatever bandwidth is used for generating the display can't be used by the CPU or blitter, that is the big limitation of the Amiga which was designed with one memory bus to keep the price down.
I don't think VGA cards were out yet but you could have had an X68000 with its dual-ported VRAM and 65536 color graphics. I think it cost around 400,000 yen.
