efficient transfer of power onto the disc will dominate arm speed.
throwing with equal efficiency, a faster arm speed will throw farther.
99% of disc golfers have power efficiency that ranks from terrible to fair (on a scale of terrible, poor, fair, moderate, good, excellent, superb).
e.g. 75mph of arm speed and a 45mph disc launch velocity sucks.
45mph of arm speed and a 60mph disc launch velocity rocks. (yes, this is possible as most of the speed isn't generated by the velocity of the arm itself).
if you really want to improve your D, i would focus on the efficiency (snap) aspect. 380-400' of golf d power implies you probably have an average launch velocity in the realm of ~45-49mph.