Yeah, surround mode has definitely solved all hiccups that were occurring. Again, this wasn't an issue specific to TestUFO; it just happened to give me a way to identify the issue - games, video playback, browser scrolling... really everything was affected.
Unfortunately, its also a giant pain to make the Windows desktop environment work "normally" - i.e. maximizing windows or snapping to display edges, etc. in surround mode. It also takes like 3 minutes to enable if I revert to standard single/multi-display mode, because for whatever reason it can't remember ANY settings like monitor arrangement, bezel compensation, preferred refresh rate/resolution, etc.
TestUFO's requirement to be on the primary monitor makes sense to me. Your mention of the possibility for monitors to be at a slightly different refresh rate brings back the notion that perhaps one or more of my monitors is somehow at the driver level running at like .0001 Hz off from the others.
Well I'll be damned... I downloaded a program called Monitor Asset Manager, and here is what the three real-time display profiles show:
Bleh. I thought I was on to something there... I used CRU to try and force the same frequency on all 3 monitors by creating a custom 120.40Hz resolution and setting it as preferred. Every time I restart the graphics driver with CRU and refresh MonInfo, I just wind up with a similar result to the image I posted above... either one monitor at 136.40kHz/119.20Hz, and the other two at 137.70kHz/120.40Hz, or vice-versa. I can't seem to get all 3 to run at the same rate.