No what I mean is the sound of a mustang comes from all the factors I mentioned,including sustain. HOWEVER, increasing the DC resistance of the pickups will dramatically increase sustain. You aren't gonna get hot pickup sustain with vintage mustang pups.
A pickup doesn't have to be a humbucker to be hot either, take the SJAG3's, they are hotter than the PAF humbuckers. So you get the true single coil sound yet your sustain increases a #### load.
Personally, i would put SJAG2's in it, they still sound vintage but are hotter and notes sustain longer. I had a set once, i gave one away to a friend and broke the other one. i'll replace them someday as they are brilliant in both Jaguars and Mustangs alike.
So the overall sound of mustangs come from all factors I mentioned but sustain is gonna be limited (in any guitar) with such low output pups (5.5k). IMO fender shoulda just put strat pups in em when they made them, they are higher output and sustain longer.