I would rather prefer to limit them to their role of AA gun but, specialy for Germany, it seems they used a lot their 88mm AAA as antitank gun (El Alamein battle, i.e.).
I like that because you are correct, in Real Live, AA was just high-velocity ammo pointed at the sky, point it at a tank like the Germans did, it became an effective anti-tank.
I like the idea of taking out tank and/or armor for the pre-combat AA roll.
I don’t know if it would be unbalanced but I think I will try this next game
I just learned that Germans put their Flak 88 on tracks and plates so it becomes the Tiger (Mark VI) and deliver them for the Battle of Kursk in 1943.
This gun seems quite an invention.