In all the tests I have seen the Mugen beats the Hyper 212 by 6 to 7C.
It is huge though and if you have RAM with high heat spreaders it can get in the way. My GSkill Ripjaws X just fits under it. I believe Corsair Vengence will not.
As far as removing the Hyper 212 and installing the Mugen goes, I dont know if I would go to the trouble honestly. The 2600k runs pretty cool and the Hyper 212 will generally get you to around 4.5Ghz. The Mugen will get you a few hundred megahertz higher but 4.5Ghz seems to be the sweet spot for the 2600k. Pretty much only benchmarking or running a system with 3 x GTX 580s will benefit from higher than that.
It might however extend the life of your chip even at 4.5Ghz. Cooler is better after all. I use IC Diamond thermal paste and my 2600k stays in the low 30s idle and mid to high 60s with prime 95.