I will offer a differing view...if you want a car that goes fast from point A to point B then a diff makes more difference that willy-waving big power figures.
I have a Quaife Diff on a 200 BHP Mk3 GTI and it made a huge difference...much more so that adding 50BHP did, where cross country pace is concerned.
But, a quaife Diff won't stop wheelspin off the line - you will just spin both wheels...it will however, kake the car WAY faster out of corners, once you calibrate your brain into accellerating into a corner.