60 Mhz should be a maximum :
http://homepage.mac.com/schrier/stats.html
I just tried the OC because i have a logic board to spare, and as far as i know the floppy drive doesn't like to be pushed too far.(you cannot boot from it anymore)
Was lucky enough to get a Raster Ops Paintboard LI with one of the IIsi's, huge difference !!