The 90/92mm fan mount holes are built into the heatsink. I believe someone over on [H] had to file down the screwholes in order to get the NF-A12x15 to fit (and also had issues with the provided fan clips).
With a NF-A9x14 2200RPM/1.5W + Kryonaut on an open bench the Black Ridge appears to be able to hold the 9900K at ~90-92'C with a 155W load (Blender Classroom, 4.7GHz AVX with -30 VOffset [1.21v load]).
With a 92mm fan I think a 130-135W Long Duration Power Limit might be possible. The heatsink does a very good job of conducting and dissipating heat (gets warm to the touch) and as someone mentioned it takes a while to saturate the cooler, but it just doesn't have enough mass or surface area to be able to handle hot stuff for too long.
With a NF-A9x14 2200RPM/1.5W + Kryonaut on an open bench the Black Ridge appears to be able to hold the 9900K at ~90-92'C with a 155W load (Blender Classroom, 4.7GHz AVX with -30 VOffset [1.21v load]).
With a 92mm fan I think a 130-135W Long Duration Power Limit might be possible. The heatsink does a very good job of conducting and dissipating heat (gets warm to the touch) and as someone mentioned it takes a while to saturate the cooler, but it just doesn't have enough mass or surface area to be able to handle hot stuff for too long.