What is the ceiling of any of the new head coaches, if Mickey Loomis is retained as GM

That wasn't the question, though. The question was about his ceiling. He hired Sean Payton, and Payton won a SB while Loomis was the Gam. So that HAS to still be taken into consideration when talking about his ceiling.
His ceiling is Super Bowl as it is a historical fact. That does not mean that it has been his recent trajectory. It also doesn't mean that he couldn't do it again in the future. He has to make the right choices and then get some luck.