Best that I saw could make 5GHz ~1.35V. It's about max it will make on water but good luck getting chip like that. 90% haswells won't run stable @4.7GHz+ on air/water with reasonable temps.
Some of them have weird clock walls .. when you have to set much higher voltage to pass next cpu ratio. Think that your CPU can make 4.5GHz 1.2V but for 4.7GHz it needs 1.4 and won't even boot at 5GHz regardless of used voltage or cooling ... some are like that while other will scale much higher without bumping voltage.
@eek142 , there was no issue with IHS, people were replacing TIM to lower temps but it doesn't mean it was so big issue. Many IB were running about the same before and after delidding. Even if temps were lower then general overclocking wasn't much better.
Issues with IHS placing can be when under load your CPU has difference between cores above ~10*C. Most chips from every generation have about 5-7*C difference. If there is too high temp difference then you can make RMA.
Max reasonable voltage seem the same for all Core series and it's about 1.35V. Intel will tell you it's even lower but they were clearly not performing long tests on their chips.
SB were randomly degrading above 1.4V. Some are still working without issues but some were dying after couple of months on 1.45V.
IB are much more resistant to degradation but I wouldn't recommend more than 1.35V 24/7. The same with haswells.