I've ran into that situation a few times in Cinebench 11.5 (my current default benchmark to run at the moment on S775). Good to know that's what's causing that behavior.
Since the original post, I've refined my understand/technique of dealing with VTT (through OCing more quads, of course). I know that CPUs like a certain... range, and it's best to try to maintain it as best as possible. Seems like 45nm quads love VTT (scales with clockspeed, and can save some Vcore in exchange for VTT), whereas the two 65nm quads I've tested are not the least bit interested in any more VTT above the stock 1.2V at around 4Ghz (maybe I'm not at the point where VTT matters for them, maybe they actually need loads of VTT and I've been mistaken, not sure).
I suppose my third question in hindsight is a bit silly - why wouldn't you want the lowest VTT you can get away with? Still, if anyone has an edge case where higher VTT for lower ratios is beneficial, let me know.
As for the first two questions, I guess I'm asking about something few people know about... :/