This kit is really what's known as a "Halo" kit, and not all configurations will take kindly to running these speeds, least of all out of the box.
The Extreme is only officially validated up to 6400MT. Not all CPU and boards will be able to 6600, especially on 4 DIMM boards. There isn't a magic bullet with these things, you'll need to tune methodically and possibly reside to running a lower frequency.
The CPU System Agent and Memory Controller VID can be sensitive to voltage changes.