I have had a similar question, with a small difference, for many years.

My question is:

"Using only s single glider and changing only the max flying weight - adding ballast shot bags, whatever - will that glider give its pilot a greater potential thermal climb rate when heavier or lighter?"

I am aware that higher weight will alter airspeeds but that is not my curiosity - other than a higher weight will raise stall speed some - which may add difficulties for the heavier glider using very narrow thermals - turn radius varies as the square of true airspeed, etc.