If you add an aftermarket heatsink, you should upgrade your exhaust fan as well...
Recently realized that the fan that came with my Xigmatek Dark Knight blows across the heat sink about 90 CFM, but it blows straight into a stock exhaust fan on the back of the case only six inches away that's only about 40 CFM. That can't--and didn't--work very well. The exhaust had better pull out air AT LEAST as fast as the HSF is blowing straight into it.
So, by replacing the stock fan with a 90 CFM fan, along with adding two exhaust fans on the top and another intake on the bottom, I was able to reduce temps in my case by 18C idle and 20C load.
NOOB DISCLAIMER: This advice is from a total NOOB, use at your own risk (would love some validation from any pros who read this)