This is an old issue from way back. There are no elevation limits due to AI constraints. If vehicles couldn't shoot straight up the AI would have to learn how to distinguish when to provide cover from what distance to a particular building in a particular situation and hope, of course, there are no obstructions etc.. etc..
It does sound like a daunting task but I prefer they would provide more of these types of fixed than pumping out modules of new vehicles. Then again, modules make money..AI tweaks don't.