What makes a pro body?

It seems to me there are certain things which are reserved for pro bodies; for example fast frame rate. Lets do some math: DX crop on d800 is 16MPx, which is 2.25 times less than its full resolution. Since at full resolution D800 can push 4FPS, there is no electronic reason why it could not push 9 (4*2.25) fps in DX mode. So the real question is if this is a software limited issue or does the D800 shutter can't shut that fast? If i was nikon, i'd purposely design d800 shutter to not be good enough to work at 9 fps; because its not all that hard to change the firmware to bump up max fps to 9. There were custom firmware updates that gave video mode to d700... D800 can be made to compete with D4 IF is shutter/mirror hardware is good enough... These are the "pro body" things i am asking about.

EDIT: whether d800 can also be made to re-focus / re-meter at 9fps is another question. I bet it can't