Hello.
From your own Doc, the max payload range is around 2500nm for the <115 660 kg/PW2040> MTOW variant. In AWS, the range is 2470 for Max payload. Not Volume-limited payload. Two different numbers to base your numbers on. Also, at 3100nm with the PW2040 engines, the payload is 32,583kg. Considering the max payload for the aircraft is 38,260kg, not sure where the "significant weight penalty" is coming from.
Also, just so were on the same pdf, that Doc is one of the data sources used to compute the 757PFs data, among others.
Talentz