You can ignore the useless WEI rating as it won't be 7.4 or 7.5 with the new RAM if it currently is 5.5.http://www.anandtech.com/show/4503/sandy-bridge-memory-scaling-choosing-the-best-ddr3
Compare the results between DDR3-1333 9-9-9 and DDR3-1600 9-9-9. Then decide if the minimal performance improvement is worth it to you.
On page 6, the reviewer wrote "The results weren't very stimulating, were they? Just as expected, gaming with faster memory just doesn't make any notable difference. I could have potentially lowered the resolution and settings in an attempt to produce some sort of difference, but I felt that testing these games at the settings they're most likely to be played at was far more enlightening. If you want better gaming performance, the GPU is the best component to upgrade—no news there."