I don't really see why it needs changing at all tbh. It's not like the soviets get access to pintles on their tanks (doctrinal IS-2, M4C and ISU-152 notwithstanding) or light vehicles to otherwise fulfill AA purposes, so in team games it fits a niche nothing else will. If it is the opinion of the community that the M5 is too good by itself than sure, but so far I haven't really seen that to be the case; everyone just wants other AA to be on it's level, which I don't really agree with in any case.
Quad shoot down planes way too fast, faster than any other AA, it can shoot down loiter planes before they fire a single shot. It is as simply as that.
Since it is one of the few AA soviet get one can simply lower the MU cost and lower the performance.
By the way the quad is a solid unit even for ground fights.
Actually planes need an overhaul and replacing the critical with HP was a step in right direction so it should get the go ahead.
I would take ti a bit further and make planes harder to shoot down but have the take "suppression" reducing their effectiveness.