[PlasmaKrab]

What is the altitude of most of (all?) hovering attack helos?
Look at their field of view in the game and you'll see the helos fly a perfect nap-of-the-earth pattern.

Even tanks can fire at elos if these stay at the same place long enough. And most tank guns can't elevate above +15°.

I don't know about max ATGM altitude, although I guess at 100m above ground you have no more ground effect portency (particularly with such a small, fast carrier-body frame), so you can climb as much as you want providing the sighting units allows you so. What would you do to fire uphill otherwise?