Because of the way programs are currently designed, you can't go any faster. The advantage of 8 cores over 4 cores is that you can do more stuff while you wait for other things to finish, but some things just don't get any faster when you make the jump from 4 to 8 cores.
Likewise, QT Pro only uses 1 core in my dual-core computer. It doesn't get done any faster, but I'm able to do other stuff while I wait.