XMP Certified is a marketing gimmick. Most memory of speed 1600MHz and above will have an XMP profile embedded in the memory. This does not improve stability, it only meanse that you don't have to manually enter the memory parameters into the BIOS, as the XMP Profile contains all this information.
Running higher clocked memory at a lower speed does not improve stability either. Memory is designed to run at a certain speed, and that's the speed it should be run at.