Minimum sink speed and minimum sink rate should provide some clues.
That data should be available. The stronger the thermal the more pilot
technique overrides small difference in glider sink. In large weak
thermals almost nothing will prevent the lower sink rate glider
climbing better and topping out higher. That effect is obvous, and
frustrating to the low pilot, at contest pre-start gaggles when the max
start height is greater than top of lift.

Andy