I have a quad core, and the speed gain that I get when I go from four to five threads is very, very small. When I switch to six threads, there is no speed gain any more. I would assume that you don't need much more threads than you have cores (virtual cores from hyperthreading included) to get full speed.

A ship in port is safe; but that is not what ships are built for.
Sail out to sea and do new things.
-"Amazing Grace" Hopper

Avatar image of me done by Chidori.