So, Vin did do some testing with an RME PCIe card, and also did SONAR vs. Cubase testing on both OSes. His findings are here.
His PCIe card performed more or less the same between Win 7 and Win 8. He also found that Cubase and SONAR performed the same between the two operating systems.
So, my poor PCIe results on Win 8 appear to be driver related. My FireWire results could be a problem with the driver, FireWire (this is still a controversial Win 8 topic), or both. Vin says he might test a FireWire interface to be sure.
Also, as we both suspected, Windows 8 does not improve SONAR performance- but doesn’t appear to hurt it either.
I will re-test my UltraLite and PCIe-424 when/if MOTU ever releases Windows 8-approved drivers for them.