How good is Mayweather in cutting off the ring?

Totally agree.
The key in this fight is Mayweathers lateral movement defensively when Khan comes forward. Floyd is not always totally flawless is this aspect of his game, as he sometimes waits to long to create new distance (pretty much his only defensive weakness). This is where Pacquiao would potentially cause trouble as well.
He has been excellent at this in most of his recent fights though.

 

Khan did that exact same thing in the fourth round. He was attacking Diaz, moving towards him, prods out a range finding left, Diaz then dropped him. Hardly an effective defensive mechanism when you are doing with all your weight on the front foot.

A skilled boxer like Mayweather or Rigo, doesn't even need to throw proper jabs as a weapon or a range finder, they use then to draw fighter in and then counter. Khan can't do this because he doesn't know how to counter.

Khan has so much natural talent but in terms of ring IQ he is a journeymen. If you took away his speed, athleticism and power he wouldn't be world class at all. A truly skilled fighter like Mayweather or Hopkins, even when past prime can use skills to win fights instead of physical attributes, which is all Khan has.