The driver of the car has a lot to do with it too.
So suppose you're ringing it's neck at 80mph+ in 5th, what next?
Christ, I like my GTD, on the roads around the Peak District today it was a blast, and plenty quick enough, but once past 70 - 80 mph it builds speed quite slowly. If there's a way to effect that then I'm all ears, but I suspect you're placing a bit too much importance on that little bit of extra torque. Torque is needed to overcome gravity, but weight plays a pretty important part too.