This might help out in the explanation of Dual cores, enabling and disabling etc , it even says that using one logical processor in some applications will speed up processing in some applications...

Also heres a link to a thread in CG talk Called " How fast is your PC with Maya 7.0" that the guys in here appear to disable hyperthreading for a faster render time...

But I just use the "use all available processors" and increase the number of threads for mental ray to use to 2, this gets my dual core PC laptop, going at 100%

Horses for courses me thinks.