You don't have to be an aeronautical engineer to comprehend that a converted pax -400 will not carry the same load as the pure freighter. If it could then Boeing could shut down its -400F production line.
The converted airframe is heavier and more restrictive; quite obvious just by looking at the upper stretched deck, by looking at all the window plugs and by looking at all the deactivated pax doors. The typical payload "penalty" is from 8 to 15kgs [1000s], depending on specific modification certifications. Although, an exhorbitant fee can be paid to Boeing for "paper" recertification of higher payload cabability.