Will I notice any more overclock headroom?
Probably not. Same power phase design on both boards. (More phases would possibly mean higher, more stable OC's, but it's the same on both)
Also, that's for CPU's anyway. GPU OC'ing is NEVER dependent on the mobo.
Will it run cooler?
Almost definitely not, but it wouldn't run warmer either. Once again, GPU temp would NEVER be affected by the mobo.
As far as I know, and please correct me if I am wrong - the Z77 only adds PCI 3.0 for Ivy Bridge chips.
That's correct, although I think the P8Z68-V LX can too. (My P8Z68-V LE can)
I would just sell the the P8Z77-V LX. No real reason to change to that board.