The U-2
Is an odd duck, light, very high aspect ratio, and plenty of Power/weight, it routinely operates at very high altitude, frequently nibbling for hours at Stall, overspeed, upset, etc. Coffin Corner is quite familiar to its pilot. It is common for overspeed and Stall to be separated by less than two knots. Not really an accurate comparison. I don't recall in the 447 thread, (either one) where CC issues were completely rejected?
rgds
bear