I'm interested in this as well. I also have an i7 2700k and when trying to o/c to around 5GHz the voltage goes up to about 1.5+. It seems stable enough when priming and the temps go to about 70. I have a good cooler (Noctua DH14). Temps are ok I think but I'm just not comfortable with the voltage. When I tried to cap it, it BSODs in prime95.
In the end I've simply decided to dial back the over clock. Now I run it at 4.64GHz and the voltage peaks out at 1.35 GHz in prime, and the temp peaks at 53. This is fine and I'm comfortable with it.
I really don't know what determines the relationship between clock speed and voltage in the bios. I know that the bios automatically increases it for higher clocks but is it somehow calibrated to what your CPU is capable of, and will it differ from chip to chip?
After a LOT of experimenting, I've concluded that my over clock to 4.64GHz is pretty much the limit of what my 2700k is capable of without getting stupid temps and voltages. I'm both happy and unhappy with this. Happy because it's still a fast CPU. Unhappy because by choosing a 2700k instead of a 2600k, I was hoping it would be a binned part that would be capable of more. I know nobody really knows whether this is the case (except Intel, and they won't tell us), but my experience tells me a 2700k is probably just a rebadged 2600k and that I happen to have got a poor one.