Thing is that a weight change that substantial would have a very conspicuous effect on speed at that altitude. Induced drag at that altitude is a pretty large component of overall drag (relative to sea level). There's 3600 lbs of fuel remaining on that bomber in WW's screen shot, and its only good for 18 mins of full throttle flight. You could imagine the difference between 100% and the current fuel state.
Does anyone know the wing angle of incidence for the B-29?