I had something like this happen. Sometimes they program different XMPs or simple forget to. XMP means nothing, it's just a profile, your RAM will do it's rated XMP settings just fine, as it's just an overclock (anything other than 1333mhz CL9 is overclocked RAM).
See the 996902 on the right, I had the RAM, had it's XMP profile just fine of 2000mhz cl7-10-8-27. I RMA'd it, and the RMA did not have the XMP profile put in. I'm pretty sure that Mushkin custom made this RAM for me, as it's EOL and I insisted if they could find it in their hearts to give me PSC instead of BBSE 2000 CL9s, and they obliged.
I'm using 2 separate kits of RAM as well, but they both using identical RAM ICs (only differnence is one is a week older). They are both PSC XDZs, 996902 2000 CL7-10-8-27s, and 996826s 1600 CL6-8-5-24
You don't need to run RAM at the same speed, if your motherboard is capable, you can run them at different timings for each channel. My 2 kits are binned differently but the difference is one will crash at X timing while the other will simply error, they aren't different enough that I could do entirely different timings on each channel. Now, at 2400mhz they are different, but my IMC isn't good enough to do 4x2gb 2400mhz.
DDR3 is good for at least 1.75v on air, if not higher (i'd only max at ~1.8v if it's a rare, end of life ram like PSC/BBSE, not some generic mushkins) and ~80*C (won't even hit 50*C with just 2 sticks at 1.8v, 4 sticks gets warmer but they'll hit just 65). You should crank up your voltage, see what you can do. Rule of thumb I'd go by is a single jump up in frequency warrants a loosening of timings by 1-2, depending on how large the jump is.Edited by Belial - 5/19/13 at 2:21am