I have been clocking my 2500k on P8Z68V at around 4.5GHZ at 1.3500V with a Hyper 212 EVO. Below are the snaps of the tweaks in the BIOS when at 4.5GHZ.
It idles at around 41-43C at 1.6GHZ however, when I tested P95 Tests at 4.5GHZ for 6 Hours without any issues. I got temps of about 75-77 C...