4.6 needs a lot of voltage on my 3570k.
4.5GHz needs 1.28V
4.6GHz needs 1.32V
4.7 needs 1.38V which is the max I'm willing to go.
Temps are awful at 4.7 (can go over 90), bad at 4.6 (over 80) and just dandy at 4.5. 4.6GHz is fine in normal use, so is 4.7 but on benches it's easily over 90/80, which is fine but, 4.5 is 73 max in stress tests.
The voltage and temp trade off for clock speeds gets worse and worse as you go higher.