Wacked, there is more than one 550...one has 13300, others have 13800 as mtow. The Bravo has even more and is called a 550 as well.
And right, a 501 could not possibly have more than 12500 (FAR23/25!) and from a fading memory IŽd say that 11500 sounds pretty correct. Your thrust figures are correct also...(JT15D-1 vs. D-4, is it ?)
Overweight isnŽt the issue in this accident I would presume. Might been a contributing factor, but if so, IŽd doubt that there would have been much of a difference to max T/O weight...