I don't think it would be broken, but also think that what it really needs is to have the AA ability improved. At Vet 0, it seems almost worthless for AA. At Vet 3, it sometimes shoots down planes in a their first pass. I've watched it in comp stomps and seen it not fire at planes or unable to rotate the turret fast enough to fire at planes. Maybe just moving some or all of the rotation bonus from Vet 3 to Vet 0 would make this okay.
That said, it is interesting how this thread has so many replies yet there isn't any similar thread about the USF AA halftrack. To put it in comparison, for 30 more mp than the Ostwind, you can purchase a USF AA halftrack that has 1/2 as much health, 1/7 as much armor, and moves slower (at least in game - IRL the M15 was almost twice as fast). Even at that, people complained when the USF halftrack had a 360 degree firing arc so now the main armament only fires to the rear.
I might not have mentioned it on this thread, but ALL DEDICATED AA units should reliable kill airplanes due to how they work right now. Quad, AA/Flak HT, Ostwind, Centaur, Bofor/Flak Emplacement (not HQ-not base bunker), should have similar performance.
I think that anything done to the Ostwind should be on top of having their AA buff to match other similar units.