It's been awhile now but Delta operated two MD11's with two aux tanks mounted under the floor. Total additional fuel was 26,000#, which of course came right out of the pay load. These two aircraft were dedicated to the LAX/HKG non stop which on occassion would exceed 15+ hours. Very hard to do this aircraft and make a buck with the restricted payload. They actually use to fuel the airplane with scuppers under the wings so as to catch the fuel as it topped off with the overfill breakers pulled.
The MGTOW on these aircraft were 628K and I believe there were further gross weight increases as the aircraft matured.