Maybe just a few percent difference in faster RAM. I have 8GB of DDR3 1600 Cas 7-8-7-24 and it gives a 7.8 WEI. At 1333 cas 9 ( default timings) it gives a WEI of 7.6.
I'd suggest the ASRock P67 Pro3 board. Its performance is quite impressive, and it's a good deal if you don't plan to do SLI/CF.
DDR3-1600 CL9 is Sandy Bridge's sweet spot. I run 12GB (one 8GB kit and one 4GB kit) of G.Skill Ripjaws X DDR3-1600 CL9 in mine. I get RAM WEI of 7.8 with mine -- not that I consider WEI a true measure of performance.