Awesome. I think I'd just prefer to make his plate armor specially crafted to fit on top of his studded leather armor. Mechanically, the plate armor basically wouldn't fit properly without the studded leather beneath it, and he'd have to lug around the combined weight of both whenever he wears it.
If you think it should have a higher armor check penalty than normal plate, that would make sense also. He of course wouldn't add the armor bonuses together or anything stupid. He'd only get the highest one (plate).
I'll just leave the cost as-is, since the Quick Escape for the light armor would no longer be needed, and it seems like a reasonable cost modifier for the ability to "combine" armors as well, I think.
Sound OK?