You didn't tell us how you obtain the temps.
If the temps were at 100° C, you should either observe throtteling or shutdowns. If you do not have any throtteling, then your temp readings may be way off.
Sorry, I didn't know to provide this info. Yes, I did observe throttling down to about 2.5ghz (my guess is that it somehow was able to dissipate enough heat once it throttled that much that it didn't need a heatsink anyway).
I just finished re-seating the cooler and my temps are down to around 33C idle, which is in line with my old i5-2500k. Running prime95 with small FFTs, it averages around 60-65C.
Upon closer inspection, it looks like I didn't have the cooler screwed in properly, because one of the mounting holes on my Asrock Z87 Pro3 motherboard was off by almost 3mm. I had to drill out the backplate for the Mugen to get it to fit.
I have half a mind to send the motherboard back to Asrock, but its working now anyway. Aren't they supposed to follow strict specifications from Intel for those mounting holes?