I think it has to do with he gearing itself and the design. Or the ratio of the angle drive (Not sure if the slingshot is 1:1 or higher). I have just read up on a few people that have swapped them out in other vehicles (not just slingshots) where they noticed no difference or better performance with out changes to any other part of their vehicles. It might be due to the change in angle of the drive drain. Not really sure but most of the things I have read is that even the heavier drivetrain is more efficient than the angle drive.
I know that Kyle D did some testing of the power of a stock NA slingshot here, Truth about Power and on his dyno got 163whp and 165tq. if the HP at the flywheel is 173 hp as polaris gives in the specs then 163 at the wheel is less than a 6% loss - normally with a regular drive train that number should be closer to 15%
if these numbers are even close to being correct then it would seem the angle drive is very efficient