Let's just assume that after the ARW the North and the Deep South split, with the latter becoming either independent or remaining under British control. Assume that history develops in a way that makes US expansion past the pictured borders infeasible. What is the highest population this US can have by the modern day? Does 300 million sound possible, or does that seem totally unrealistic? What is the highest population to where they can still enjoy close to the living standards that those in the US have today?