If I understand the question correctly, is the takeoff prepressurization increasing the weight of the aircraft?
885.9 cubic meters at sea level represents 2392 lb.
To pressurize at -50ft you need to pump up an additional 4 pounds of air, if I get my calculation right.
Closed system or not, you need to accelerate those additional four pounds during the takeoff roll so the effect is real, but totally negligible.