faster ram does not give a linear performance increase. If you don't have enough to feed your CPU then the speed increase is pronounced, but there are rapidly diminishing returns which diminish to pretty much nothing. We are currently at that point, average DDR3 is fast enough that a speed bin increase gives ~1% increase on the fastest CPUs out there.
We need more cores, faster CPUs, and/or GPU integration to push the need for faster RAM again. Until then it is better to focus on quantity of RAM, the more the better