bearfoil, on the second issue I took issue with the implication in your words that the mass had a great deal to do with the "heavy" being able to trade velocity for altitude. In a vacuum it had sufficient velocity to loose 300 miles per hour to gain 4000', if I remember the numbers in question. A 50% decrease in mass would not change that at all in a vacuum. In air it might change it a few percent due to the imperfect energy transfer. Some would be lost to air friction.
A balloon, if you could get it to mach 0.82 would lose way more than 300 mph to climb even a fraction of 4000' due to the wind friction of its shape. But most aircraft designed to fly up there can probably make that trade pretty efficiently. So declaring the mass had anything to do with it suggested you thought a lighter plane at the same altitude and velocity (on its own power) would not be able to make that full tradeoff.
I guess I was being nitpicky with some wording that set my hairs on edge.
I spent good money for those physics classes.