XMP will set your first, second and third timings, and also the voltage and frequency. (and maybe fsb also?)
I have a 2400mhz quad channel kit, that i sometimes run at 2133mhz with tighter timings (not a huge difference either way besides load on the IMC).
I've found that manually setting my first timings to that of the XMP profile, and leaving the second and third timings up to my asus rampage IV BE motherboard results in a far more stable system.
Also, using the xmp profile for 2400mhz with 10-12-12-31-2T timings, and 2133 with 9-11-11-28-2T timings results in virtually the same benchmark results, but with my cpu (4930K), if i try to run 4.3ghz or higher with 2400mhz frequency (regardless of XMP or not), my vtt voltage goes up to about 1.3V (auto set by motherboard - which is above recommendations for this cpu).
Basically I can get similar performance from the RAM and higher o/c on my cpu by avoiding XMP, due to the quality of my IMC, ie. my system performs better/more stable when i ignore XMP.
Edited by anti-clockwize - 1/28/16 at 8:27pm