Hi Pete! Great job!
Interesting on Day 1 that SNR was highest for the XG-91 on high VHF channels than the CM4228. Any thoughts why? Perhaps the higher elevation on the XG-91? Mclapp's work with DIY 4-bay's suggests that reflector width is critical for high vhf reception. A continuous width of 32 to 36 inches is needed. The old CM4228 meets this criteria but not the HDB8X which perhaps explains differences in VHF reception between the two panel antennas.
On day 1, the HDB8X picked up three UHF stations missed by the 4228 while the 4228 picked up one unique UHF station missed by the HDB8X. SNR's of stations received by antennas were generally within a DB indicating similar UHF reception (as you stated).
The Day 2 results were also interesting. As you know, an 8-bay typically has 3 to 4 dB higher gain than a 4-bay. Based on your testing, performance of the 4 bay dropped off more than one would expect with lost reception on several stations.
Awesome, thanks for doing the work and reporting!
All the best my friend!