Mahomes would have probably gotten the Cowboys to a superbowl right now. He wouldn't be as good as he is with Reid.
The problem is Garrett would still be the coach and he would have held him back. There is a reason the guy isn't coaching anymore. It's similar to Dak, Jerry is in the mind set he's smarter then everyone else. When it doesn't work you find out the guy was only good enough for Jerry.
Romo was 3 times the QB Dak is and Garrett held him back. While Mahomes is a much better QB then Romo was, Garrett would have held him back too.