Changing voltage can possibly improve stability, but it will do nothing at all for performance. Since performance is the problem here, there is no reason to think than changing the voltage will fix it.
Something must have happened to your memory ratio or timings. Get a screen cap of memset and whatever you are using to benchmark with just two sticks install, then do the same with all four. After that we can compare the two and see what, if anything, can be done.
...rightful liberty is unobstructed action according to our will within limits drawn around us by the equal rights of others. I do not add 'within the limits of the law,' because law is often but the tyrant's will, and always so when it violates the right of an individual. -- Thomas Jefferson