I like the idea of a multi-tiered AA system for PS2, but I think that limiting
all AA so that it only affects low
or high targets is a
BAD idea.
I say this because not only does it feel like an arbitrary limitation to put on the longer range AA, but more importantly the larger-scale battles will always have a mixture of both low and high altitude aircraft, and if every form of AA out there can only hit one or the other you've effectively cut the amount of AA out there in half for any given pilot flying in the area. So you'd have to setup twice as much AA as would otherwise be needed if an "either/or" tiered system weren't in place.
So, rather than forcing AA to choose between one or the other, I envision more of a 3-tiered system, where all AA is effective at low AA, only some of it at mid-altitute and a very select few can hit high altitude. But the high-alt AA would also be effective at low and mid alts as well. So, assuming, say, a 700m flight ceiling, here's what I see:
0-300m - Infantry general AV , vehicle mounted machine guns (which IMO need to be much more effective at AA than they are in PS1), cerb turrets (w/ similar range to the cerbs in PS1).
300-500m - non-upgraded wall turrets, Tank/other vehicle mounted AA, Infantry dedicated AA weapon (think of a 3-shot disposable SAM, like a guided Deci only effective vs air), AA MAX lock-on weapons (all 3 empires would have this available)
500-700m - AA wall turrets, Skyguards, AA MAX flak-based weapons (all 3 empires would have this available)
EDIT: Actually, now that I think of it, as Nitro pointed out I like the idea of cloud cover being a natural way to cap the height at which AA is effective - at least lock-on AA - since maintaining a lock would be more difficult once you'd lose sight of the target in the clouds. However, you could still maintain the lock if you guessed the flight path of the evading aircraft corectly and your reticle still remains red. This would mean that FaF-based AA a la the SparrowMAX should probably not be in PS2 (but I don't like FaF to begin with so I wouldn't want it in PS2 regardless)
So flying into the clouds would help but wouldn't be a guaranteed way of escaping. And flak-based AA could still shot through the clouds just fine of course.