Zombie, it's not that hard to take over the world with just 3 or 4 air transports total, filled with infantry. Just put the trans near a city, unload 1 inf, rinse and repeat. I do agree with your point that SM isn't just bomb spamming though.
For the same reason, Mr 7, buffing the air trans is unlikely to work. If air transes were buffed such that they'd attack and defend first, the easy way to get around that is to unload units next to cities and never actually attack with it. And to get around the defencive problem, just load a bunch of infantry to protect the trans (which should already be on there to take cities).
On your AA point, it's clear you've thought out the Math, but unfortunately, 28 offence doesn't work like you may think it does. Fact is, when you pack that much offence into a single unit, it will behave extremely unpredictably due to the 7 HP. Consider what would happen if the AA happened to roll less than a 7 (the bomber stays alive), and the bomber on offence rolls a 7 or 8. The AA would be practically useless. I've done some tests on way overbuffed attack/defence, and it's just too unpredictable to be feasible. It may look great on paper, but it's very different in practice.
Rather than trying to buff the AA to oblivion, I think it'd be simpler to just decrease the cost by 10 or 20. You said you wanted cost effectiveness, so the easiest solution is to decrease cost.
PS: AAs are not transportable as far as I know. I'm not sure if they can create walls, either.
EDIT: As to your claims of SM's extreme strength, it's not as strong as you think it is. It's way OP against tank strats, other SMs, and any expensive strats, but to major producers like PD, IF, or Imp, it will fall to gigantic infantry stacks. An SM will rarely be able to use all of his reinfs due to the cost of bombers and air transes, while an infantry spamming strat owning just half a continent can spam over 50 infs every 4 turns, for a couple thousand bucks. This means that if an inf spammer expands early and quickly, it can slowly overpower SM just by defending where necessary and attacking when the SM goes bankrupt.