The mouse getting less accurate can be a side effect of lowered frame rates. Check and see if Vsync is on. SC2 does not always react nice with vsync.
You can also try with crossfire off since maybe the game has issues with it.
Your cpu should be more then a match for this game. even more so with that overclock.
XPM overclocks because there is not a memory multiplier available to allow stock speed with the faster memory. it then overclocks the cpu. You can just switch it off and use the default speeds of 1333 with the 1600 timings(or in many cases even tighter timings) as long as you set the voltage correctly.
My memory on one system(I5 750) is 1600 @ 7,8,7,20 and running 1333 @ 6,7,6,20 since the day I got it without issues.