Overclockers Club published a review on the ASUS ROG Maximus VI Extreme
A quote from the article:
When you look at the performance delivered by the Maximus VI Extreme at stock speeds, it is going to deliver performance within a few percentage points across the entire test suite. Really there were no surprises here with the stock performance. When overclocked, the M6E delivered performance increases across the board making for a truly inspired performance. On this board I was able to eek out another 50MHz of bench stable performance out of my 4770K thanks to the flexibility and the granularity of the voltage options in the Crash Free BIOS BIOS at up to 4749MHz by manually tweaking many of the options, even though the auto rules for all three of the Level Up options are robust enough to get clock speed levels of 4.2, 4.4, and 4.6GHz stable as long as your chip can run the number. If manually tweaking is not your strong suite, you have the previously mentioned Level Up feature in the BIOS as well as the 4-Way optimization tool in AISuite III.ASUS ROG Maximus VI Extreme Review @ OCC