I have the p6x58d Premium. I have 12 gigs of mixed ram (timings very close to each other) and I have it set to xmp profile and it works fine. I don't overclock though. 6 x 2gb 7,8,7,20 at xmp half Kingston half Corsair.
Intel recommends caution when populating all the slots, and/or with 12GB RAM on a X58 platform. XMP is to be used with 6GB only, otherwise the integrated memory controller may not handle the extra load. That´s why I always downclock the ram frequency with 12GB. You should do the same.