I know it's difficult to calculate but in my recollection, it was the launch rate which always used to make a difference between a VGS and a BGA/GSA club.
With a VGS, there was always someone there 'chivvying' the cadets to ground handling the gliders and doing other things [like positioning the cables, hanging on to wingtips etc] and in general making the operation more efficient whereas with the BGA/GSA clubs I've visited there were always those who were reluctant to assist, sitting in the caravan waiting for 'their' flight.
At 613, we usually operated AEG with 2 or more aircraft, when a gllder launched it was quickly replaced by the next one to be launched and as soon as a fresh cable arrived the next cadet would be airborne; by spot landing you often got a turnround time of less than 5 minutes with the pilot not even getting out of the aircraft, just a change of cadets. I never saw this at a BGA/GSA club which to me seemed to operate in a very slow and innefficient manner,