Changes to the SAM seem like a worthwhile idea to test out, but I don't think they are oppressive at all.
SAMs are already one of the structures you really never want to build because 800 mass is a lot, but they are necessary sometimes as SAMs are one of the few things able to reign in the oppressive T3 air, if just a bit.
Changing the damage to 1000 is a great break point for T2 air though and from some quick looks at the unit database, still results in all strats dying in 4, all ASF in 2 salvos, same as they are now.
So how about instead of increasing the SAMs cost, decreasing it so that defending against incoming T3 air is slightly less punishing. Something like:
Build time: 1195 -> 1100
Mass cost: 800 -> 700
Damage: 1200 -> 1000
Hitpoints: 7000 -> 6000
Changing it like this would make SAMs worse against T2 air and better against T3 air, which seems all around desirable.