There will be little, if any, difference between 2 X 4 and 4 X 2 sticks of identically timed RAM. The reason to go to sticks of 4 GB is that you can populate your mobo up to 16 GB vs. only 8 GB.
RAM is the last place to seek performance enhancement. The difference between a rig running 1333 GHz and 2800 GHz is miniscule. Yes there is a difference, but the greatest performance gains are in the speed of the processor and HDD/SSD.