How does this work with Monks and Barbarians? If it stacks its a great benefit, but so is picking a Dwarf and getting armor for your wizard. I'm not sure its that great a benefit, as these are two classes that I saw a lot of warforged come from. Just like a lot of racial abilities, its not always great for every class and some wont get used at all like the mountain dwarf.
I was thinking it didn;t stack, but its better than the Mountain dwarf because it cant be taken away.
I think the final solution is that you can pick what armor type you want based on the subrace (or model) as you suggested.
+2 con base
No armor variation + 1 in any stat
Light armor AC 14 base + 1 in Dex
heavy armor model with AC 16 as a base. +1 Str
Feats such as mithril or adamatine would upgrade this by 2, so mitril would give AC 15 and admatine would give AC 18. ( i would look at the armor proficiency feats and model warforged ones after those and maybe add a kicker ability)
obviously numbers would need to be crunched to find the sweet spots for the bonuses. is AC 14 too good? ok 13.
is heavy armor not that great (well by itself it probably wouldnt be but with the other bonuses maybe it is and doesnt need +2 str like the dwarf)