re: How much more success could coaches have at Bama than at other schools?

While you're wondering that, wonder this:
How much more successful would Saban have been at LSU if Florida wasn't the Tigers permanent opponent from the SEC East?

Both Alabama and LSU face Auburn on an annual basis. But only one team in the SEC West has to face the Gators year in and year out and that is LSU. Barring that Saban would have had a few more wins.