The trouble with an A500/600/1200/CD32 is that there is interleaved memory access between the CPU and chipset (unlike a PC or a Genesis) so I guess if they are running at different speeds the clock signals still have to "line up" at certain times. On Aminet there are some tales of hacking an A500 to run at 14MHz (2x the chip bus), maybe an A1200 could run at 21.5MHz (3x) or something?
As for the A2630, I think Dave Haynie's archives had some updated PALs that helped with overclocking if the card had 80ns or faster RAM on it.
One time my brother and I each had the same model x86 motherboard, and 400MHz Celerons. His would run at 600MHz but mine wouldn't boot at that speed, I could only get 570MHz. It just depends on the chip. Later I got a 500MHz chip for cheap and that one wouldn't go past 575MHz! And of course this was already past the good ol' days where you could change the multiplier, so the 500 was slower than the overclocked 400 because of bus speed.
