XMP just makes ur ram run at its stated manufacturing speed easily. XMP is a set of information built into your ram. It just tells your mobo which settings the ram is designed to run at, and your mobo then applies it automatically for you. SO xmp is really nothing special, all gaming grade ram nowadays have it
Most of the times it entails your ram being "OC-ed" because the cpu doesnt officially support that ram timings, speed etc. Your haswell does natively support 1600mhz so it wont be an OC technically.
Im running my 1600mhz corsair ram at xmp with sandybridge (only natively support 1333mhz), no problems. No real improvements over 1333mhz though, but it feels good to make ur equipment run faster