CL is the CAS latency, or tells you how many clock cycles it takes to perform an action (basically), lower is better (faster)....so if going to high freq DRAM I prefer to use performance DRAM at 2400 the CL10 is considered performance, CL 11 is average and CL12 is slow. With Haswell, the best I've found,m DRAM wise are the Gskill Tridents, in this instance would be their 2400/10, in 2x8GB
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231589
Not only a little better performance wise, but also about $10 less