I agree with Steven L,
The differential forces required to roll a wing must be easily of the order of 5-20 kg, making a increased tail load of "5kg" influential.
after all total aerodynamic drag of most gliders in level flight at slow speeds is somewhere around 20 lbs isn't it?

Max weight divided by numerical L/D at best L/D speed?

I.E. std Cirrus 728 lbs divided by 36 = 20.2 lbs drag (no water)

interesting topic!!