The Mac IIfx's can be clock chipped from 40MHz to typically around 50MHz. There are two basic routes to this end; Modification 1 and Modification 2 as outlined in the Mac Crystal Oscillator Speedup History File.
I recently received some anonymous information from a prior employee of Sixty-Eight Thousand, Inc., a Scotts Valley, California company that used to manufacture, among other things, the Dash 30fx. I was told:
"They took IIfx motherboards, removed the 80MHz crystal oscillator and replaced it with a 100MHz crystal oscillator. The boards were then put in a tower case with a big fan (no heat sink) and configured with whatever memory, drives, or boards the customer (mostly Pre-press houses) wanted. I can tell you that it was surprisingly reliable. Other than the fact that the Processor Direct slot didn't work, of course. ;) We had a failure rate of about 1 in 25. i.e. the board wouldn't run at 50MHz and we'd have to downgrade it. There weren't many failures in the field either. I'd be curious to see how many of the machines are still running today."
There aren't all that many data points for the Mac IIfx on the Clock Chipping Statistics page, but it's worth checking out, and please fill out and submit the Clock Chipping Statistics Survey form if you have clock chipped your IIfx so we can build up a large set of statistics.
I just heard from someone who replaced the 40MHz 68030 and 68882 with the 50MHz versions, and he is now running at 60MHz without any problems.