I'm not surprised that the tech didn't give out much info, and I'm not hopeful that you'll get much of an answer to your undervolt findings.
I think someone on the Nvidia forum made a thread specifically for GTX 560 VIDs, if I remember correctly, but that might've died. As for me, I had a stock VID of 1V, and ran 950Mhz stable at that. I just straight up pushed to 1.05V and am stable at 980MHz. I could probably optimize and drop the voltage a fair amount and stay at these clocks. 1.15V produces crashes, and I doubt I can push much further without VRM sinks.
UPDATE: I started toying with it a little bit. I didn't want to overvolt to more than 1.05V for a small overclock, so I worked my way down in volts. The lowest I could go was 1.025V which I'm at right now, with a seemingly max stable of 970 core through OCCT for a few minutes, a full run of 3DMark11, and some BC2 gameplay. I bumped the memory to 2200 for the hell of it; nothing happened during BC2, so I'm guessing it's good.
AFAIK, memory OC's don't help much with framerates; at least, not at 1080p. Am I mistaken?
Edited by EfemaN - 2/15/11 at 11:34pm