Just talked to Wayne Shaldenbrand (owns Sunsation). They saw about 1.5 mph on mine. His opinion was the skeg length is not your issue if you lost 5mph. He guesses your X dimension is not right for the Sportmaster and the whole drive is too low. His opinion was you won't see much improvement by shortening the skeg.

I think your right, have not looked at the dimension of the sport vs standard lower but that makes sense. But they look good