VPC 4+ always showed 300-400Mhz "rated" speed from Connectix's dynamic CPU/FPU emulation in DirectX, this is only showing you're obsessed with numbers than understanding limitations of the hardware emulation Connectix/Microsoft used. VPC 1-2.x shows in DirectX as a Pentium 133/166, VPC 3 displays 166MMX, etc. Less FPU usage based software= faster CPU emulation is fairly easy to understand of how it influences VPC.
The bigger question is why aren't you running Windows XP SP3 with your VirtualPC if the goal is to push your useful life of PowerPC hardware? On Windows XP you can use 512MB w/o any registry hackery to Win9x/Win2k.
What? You've lost me now. I'm neither obsessed with numbers nor endeavoring to get speed bumps with "registry hackery to Win9x/Win2k" - I've simply responded to your suggestion that VPC can achieve 800Mhz performance as opposed to the 133Mhz I'm getting - I'm not chasing benchmarks only trying to run software based on your advice.