680 is pretty respectable as it is already. I'd probably stay on 1.45v, especially with the vf700cu. 67'c isn't bad at all, but compared to water where I never see about mid 40's under load, it's still pretty hot.

when I was on vf900cu, I was also below 50'c with 1.5v. i apologize for not having much experience with those temps and the voltages to know if it's a problem or not. i did run 1.4v with stock cooler and had temps in the 80's. my o/c was around 650 or so iirc.

i'd probably stick with the 680 core and fiddle with geometric deltas. maybe you could squeeze a little more performance from there.