Don't know if there would be a sound difference but comfort might be a little better with the W5k pads as they appear to have a smoother surface than the stock A900LTD pads. Not that the A900LTD pads are uncomfortable.
Yep, you can fit W5000 pads on the A900LTD. The A900LTD pads give a tighter fit than the W5000 so that may increase the bass. I still prefer W5000 pads as the music sounds more natural and open compared to A900LTD pads.