Have you considered trying a S370 CPU in a 'Sloket'?
I have thought about it, especially as it seems my CPU is a big bottleneck even for my TNT2, but going the slocket route would add a lot of complicated, which I don't really want at the moment as I am unable to get a officially supported CPU to work!!) I would need a decent slocket that change FSB speeds and control voltage to down to 1.8 etc, and no doubt i'll get one that isn't supported or something. I might go that route eventually, as it is the only way to get to 700+MHz on my early P2B. I'm basing all this on this guide: http://homepage.hispeed.ch/rscheidegger/p2b_procupgrade_faq.html