With the 500, 1000, and 2000 you could pop out the 68000 and replace it with a 68010 or faster 68000. No need for any other hardware (I think).
Would it be possible to clip a 68000/20 over the one on the 600 motherboard in the usual reverse-socket manner without the need for modification? I'm looking at the MC68HC000FN20. $8.21 for the chip, let's say $10(?) for a socket. $20 for a 2 MIPS (so claimed by Motorola) upgrade isn't bad... provided this theory actually works.
But then there's still the limited memory problem... Sigh.