If it's anything like the x51 R1 you can't. Hopefully you have the latest update to the bios, and if anything, the R1 only detects and uses SPD speeds and uses the default speed of the module. That being said, most high speed modules have 1333 MHz hard coded as the default. Check around online for sticks that have that. I have seen a Kingston Hyper X kit that has 1600 MHz as default. I think I saw it around Amazon, and it's the one that has 4 colored heat sinks. If unsure, ask for the default SPD (I believe it's the "maximum" speed in cpu-Z, I know it's misleading). That's the best I can tell you.

* Matched memory modules. No mix and match.

* For PC3-12800 XMP memory, go into the Bios and turn on XMP. Go into the Overvoltage Config and set QPI and Uncore voltage to +200mv. Leave everything else on Auto