85'C during IBT/P95 small-FFT, 65'C during typical loads. The NH-C12P mostly only struggles with the high-end of the thermal loads like the stress tests, as during general productivity loads it hovers under 60'C and during gaming it stays below 65'C.
The 2700K operated at 90'C+ during IBT load. But that was probably partly due to the NF-P14 + LNA -- I just don't get the point of using fixed-RPM fans for heatsinks especially when they are jet engines at full RPM. I remember the same underwhelming disappointment with the NH-D14 -- just buy the Silver Arrow instead.
Part of it is also due to the gap between the surface of the die and the bottom of the IHS, which suffers from manufacturing variance. AFAIK, the TIM they use isn't terrible -- much of the benefit from "de-lidding" is realized by scraping off the glue securing the IHS to give the IHS bottom less clearance from the die.
It was mostly a cheap "side-grade" to stay true to low power consumption. You can get even smaller and higher-performance with a SG05 that can hold a 3770K + GTX 680 + H80, but that would be too loud for me.
Going forward, I don't think anyone needs more than the micro-ATX format. 2 PCI-E + 1-2 PCI should be more than enough with so much functionality being moved to the motherboards and CPU.