I have some serious doubt to the fact that you "Mastered" it a while back.
and yes it does add a lot to a boxers workout. it is great to warm up on, but besides that it increases hand speed, timing, coordination, hand eye, footwork (if used properly), shoulder stamina, there are even speed bag techniques that show you if you are throwing your punches properly. get out there and learn what the bag is useful for before assuming you have mastered it and move on. check out youtube, there are some awesome speedbag videos on there