Did the Mythbusters use someone with their awesome smart guy physique or someone actually athletic like a martial artist with a high hand speed?
Martial Artist. The best one they could find.
They also built a "robot" that they tuned to both catch at the fastest possible human speed they observed (the maitral artist) and even tuned the robot to a "super human speed" that COULD catch it.
So it's possible. And it's not a matter of starting to close your hand before the bullet it's fired it's just a matter of reacting from "hearing/seeing the bullet fired" to "closing my hand."
For humans the time it takes a bullet to traverse any distance is too quick for a human's brain to go from "a bullet was fired to close my hand." In the Mythbusters, IIRC, in "catching an arrow" the Mythbusters tried to catch it and one of them even said he tried to do it by anticpating the arrow being fired so that his hand would close in time and he pretty much still couldn't do it.
So for Superman it's possible if his reaction time is fast enough, which it is.