But this is a general descision in modeling affecting not only the Wirbelwind. You would have to introduce reload times for all many more: For example there are quite a number of plane mounted guns using ammo drums that would have to be changed.
Turret speeds however are being modeled, and the WW and OW ones had just been adjusted.
Using an ammo drum, such as the M-16 has with it's 200 rounds per gun, is nowhere near as problematic as the 20 round magazines the wirbelwind uses. If it were fired like we do in AHII, an aircraft flying straight at the wirbelwind at 600 fps (just over 400 mph, or 200 yards per second) would only be able to be fired at for 200 yards/sec * 2.3 seconds (the time 20 rounds go through each barrel), or 460 yards of aircraft travel before having to reload all 4 cannon. This means if the wirbelwind opens up at 1200 yards, the aircraft in question would have no rounds firing at it from about the 750 yard point on in. Guns that have drum magazines don't have this issue