I think A is relatively uncommon, as long as mainstream CPUs only have 2 threads...

I think A is relatively uncommon, as long as mainstream CPUs only have 2 threads. It may give you a boost when you happen to have between N and N*2 runnable threads, for N = number of cores. And those idle CPU threads were already showing up as idle logical processors in top so you would have had a good reason to try to employ them earlier already.

(terminology nitpick: HyperThreading is just Intel's proprietary trademark for SMT)