However, we're talking about very specific needs: super high performance networking. If you have that specific of a need, wouldn't you want something unfamiliar if it solves the problem best?
If it's truly better and the only difference is remove Linux and install BSD then what is BSD doing that is better/different/messed up that the packets can flow faster in BSD than in Linux?
Talking about unfamiliarity and specific needs: FPGAs are much better suited than CPUs for processing minimum-sized frames at wirespeed. They can still forward all unhandled frames to a CPU. Yes, it's a lot of development effort compared to a CPU-only solution, but considering all the kernel-optimizing-multicore-cleverness from OP I would say we are approaching the break-even point.
However, we're talking about very specific needs: super high performance networking. If you have that specific of a need, wouldn't you want something unfamiliar if it solves the problem best?