When I come by extra PEC IIIs I install them along with maxxing out the onboard memory. The only proof I have for improvement is the systems that have the extra toys don't (yet) crash after a few years. Sometimes the other BCMs need to be rebooted.
The only time I noticed a dramatic improvement was when I added the PECs, the memory, and cloned the HD to a 7200 RPM, high end drive with a 16meg cache. You could even tell the difference when you were doing maintenance. Everything seemed to "snap" when you clicked on it.
Probably not much use, but a lot of fun.
NARSBARS