In my experience with parallel code written in Haskell, hyper-threading offers only a very mild speedup, perhaps 10%. It is essentially an illusion, a logical convenience. (How long does it take to complete a parallel task on a dedicated machine? Four cores with hyper-threading off has nearly the performance of eight virtual cores with hyper-threading on.)

Many people have neither the interest nor the hardware access to overclock, and these processors have less overclocking headroom than earlier designs. Nevertheless, the hyper-threading hardware itself generates heat, restricting the overclocking range for given cpu cooling hardware. In this case, turning off hyper-threading pays for itself, because one can then overclock further, overtaking any advantage to hyper-threading.