Yeah well we are talking about a specific line sadly, I can make another poll for 670s if you'd like?
It would probably get way more hits as the 670s have a higher core and a lot more are available cheaply than the 680/690 (order of availability
The reason being is because the cards themselves are pretty different, the 670 is a lower thermal load due to having 188 shader units disabled compared to the GTX680, resulting in a default of higher boost clocks than the 680 out the gate. The 690 is two fully fledged 680 cores crammed into a tighter space so the thermals are much greater on each GPU as well as the limitations on the power input and regulation by both the PCI-express port itself and the power cables, so it clocks much lower than the regular 680.
I would have no qualms using a modded bios on the 670, I am unsure as to whether they also have a fan speed cap however my card stays stable at <=60C during a 30C day with a .025v overvolt and +160mhz core due to having a fan profile that scales to 100% instead of 85%. That helps keep the card at boost clock rates during heavy gaming/benching.
Overvolting the 670 may be a little bit 'safer' than with a 680, or not at all depending on what nvidia meant by 'sections' of the GPU being affected by electromigration when overvolting, however I would do it just cause the power usage is still less than that of a 680. Anyway its your card and I remember feeling a little uncertain when I used the KGB unlocking tool but it worked without a hitch!
I was considering bringing the voltage closer to the boards 'hard limit' of 1.215?v but nvidia's spooked me a bit and I am not a fan of being spooked, I might just swing in for the AMD HD8970 if it shows a lot more promise from the 7970 (than what the 6870 was to the 5870).
Again I haven't been running this voltage for long, and part of what nvidia said was that it would be a shortening of the cards lifespan at higher voltages, so maybe check around and see if anyone has recorded performance issues with their card after keeping it overvolted over a period of time.
EDIT: GTX 670 Boost poll hereEdited by Phirebird - 12/16/12 at 4:46am