There are three basic clock chipping issues to consider; clock chipping the base machine, clock chipping the card, and clock chipping both at the same time.
As far as clock chipping the base Mac, the presence of the PC Compatibility Card does effect the top speeds attained. The typical average top speed attained with the PC Compatibility Card in PowerMac 6100's is 76-80MHz.
I don't think I've heard much about anyone clock chipping the card itself, but I'm sure they can be sped up a little. The card comes with a 66.6667MHz surface mount oscillator at position G1.
There is a problem with the card in a PM6100 running MacOS 7.5.5 with the DOS compatibility software 1.5 such that the PC Exchange control panel will not load. It is unclear if this is a problem with 1.6.3fc3. To avoid the problem you can run Wish I were... 2.0 by Martin Blitz (mblitz@ultranet.com), and set the machine to think it is a PowerMac 6100/60, not a PowerMac 6100/80.
If you have any more info on this, please pass it along so I can update this info. I also need to go over my old notes.