But the net I took out of it was that some parts of the X protocol could be just plain more efficiently designed.
Second point/question... When using X tunneling through SSH, they offer compression. Back in the day, it didn't seem that that compression would be anywhere near as effective as dxpc. If as you say, most of what's flowing through X now is bitmaps, etc, perhaps SSH compression with modern toolkits is a better bargain than it used to be.