Mullen is a good enough coach but I think their absolute ceiling is 9 wins. They're probably going to hover around that 7-5/8-4 threshold. Still, consistent winning seasons is good for them. The fans will start to expect more though...and he'll get canned after one bad year.
Nine wins is the absolute ceiling, and that is with a laughable OOC schedule and a patsy bowl opponent. If they play some quality non-conference competition and get a halfway decent bowl opponent, the ceiling is probably seven, maybe eight with a bit of luck.
As I've said on here before, with all of the seniors he had this year at MSU, this was as good as it will ever get in Starkvegas under Mullen, and he still only went 8-5, got blown out by the SEC powers, and had 14+ point losses to Ole Miss and Northwestern. And the only thing that even kept them at that level of success was the implosions from Auburn and Arkansas, otherwise they would have been languishing around 6-6.
Again, it's all downhill from here in Starkvegas for Mullen. Goofy, little brother billboards and porous recruiting classes only do so much.