im sorry if my english is super bad.
"Would a corp buying in higher than 270 (country controlled public say) be able to produce higher?"
-Short answer: no, it's hard capped. ( maximum) The limit is close to what i posted above.
-long answer: You can already buy up to 330 supplies straight from the market in the supply tab, and from common market if there are public corporation producing at max level.
-Very long answer: Common market needs a re-haul to consider the quality that the buying corporation wants.
Producing very near to max level is what you want , but if you go above you lose so much money because you buy expensive supplies but you cannot produce higher quality.
Imagine you are filling a glass of water, and once it's full you still poor water in it. the overflow is lost.
Some people like to put one or two high quality product, and lower the rest, but it's a bit complicated.