as far as i've heard, and talked about with various figures, case porting doesnt "make" power. it is a motor response type deal. its gains are in the responsiveness of the motor. a tuned motor with case porting will be more brapppppbrapppp than the same without case work done to it. its about getting that F/A mix from the carbs and reeds into the cylinder on the smoothest path as possible. when you are pushing air at 4000+ft/s, the air likes to move the easiest way. case porting opens a riverbed for it to flow into the transfers.
DISCLAIMER: im a nub and this is far as i know.