The megahalems has wide spaced fins, so a high rpm, static, cfm fan is not needed. If you look at a true the fins are pretty close together resulting in needing a higher rpm, static cfm fan to get the air to flow through it better.

As noted before the large fins when using two for push/pull set up will only cool maybe 2°-4° better. And in some cases if you have a larger static pressure fans they will cancel out each other and make higher temps.