I don't believe in a natural limit exactly, in the sense that if you can't gain any more weight whilst eating 4500 calories, then upping to 4800 should add a bit more weight in theory. Although there is obviously a limit to how much food a human being can take in and process per day. So in my opinion the term 'natural limit' should apply to anyone that eats 500 calories over maintenance and struggles to add any lean mass. (everything else being perfect)