I was watching some old YouTube videos and watched the Sports Science video of Padraig Harrington's Happy Gilmore swing analysis:
We can see some interesting differences in the actual swing itself (sans the actual run-up to the ball).
Most notably at impact where he has far more pelvic rotation as he gets into impact:
We can see in the Gilmore swing the left leg is straighter and the left hip is 'working back.' We see a different right foot position as well.
I also thought it was interesting that the Gilmore swing in the startdown goes into dual external rotation of the hips whereas his old swing does not.
So while the run up of 4 mph may have helped create some more force to improve Harrington's club speed by 7 mph, I think the hips and pelvis in the actual swing play a larger role in Harrington's increase club speed.