I once did some research on this too. It's been a long time so I can't recall all the details. But I recall there were logic and/or timing issues as well as the voltage restrictions with 040 cards that twarted just plugging in an 060 in an 040 space. But I'm no authority, so if some one else has more info, I'd love to see it too. And off the subject a bit, this site has alot of Amiga CPU info for overclocking if you haven't seen it already...
Amiga OverclockingPlaz