My 3600, with 16gb of GSkill 3200/cl14, had trouble hitting even advertised mem clocks in the original bios my ASRock Extreme4 WiFi came with(1.10 I think), but I updated the bios to 1.60 and was able to hit 1900/3800 at clock15 on the next boot(I just picked speeds and bumped vdimm to 1.45/SOC 1.1, with GDM Disabled, bios set timings).
I had an issue with my old Seagate HDD, so I dropped mem/fabric to 3600/1800 thinking that was it, it wasn't, the old drive was just failing....
It's still sitting at 18/3600 right now, I haven't gone back in the bios since I swapped out the drive to an Evo 970.
I also haven't updated to the 1.70 bios for my board, it's 1.0.0.3ABB, and some folks are saying weaker boost clocks with it.