I don't think it's a good idea to make AA units agasint CAS.
Sure, you will bring down planes but mostly AFTER strafe.
Once I had a game where my AA HT brought down around 20 planes, 18 of them after strafe but in fact it only helps your opponent, why?
Because plaes are flying to you, so if you shoot them down, they will crush on your side of the map.
In that game I mentioned, I lost more units to plane crahses than to enemy so...
PPsh is not bad upgrade in early-mid game but in late game when closing in means losing 2-3 models you must relay rather on standing around the corner rather than running in front of LMGs blob.