He probably wouldn't improve as much as base naruto does when he enters SM simply because SM doesn't compliment minato's abilities as well as naruto's (for instance, minato doesn't have a nuke and he already has instant movements). It would still make him a lot more powerful though, and unlike Pain arc naruto, minato would be able to fuse with ma and pa for indefinite SM. Or he could just warp himself or a bunshin away whenever he runs out.
Anyways, if he could fuse with ma and pa and use the mode indefinitely, he'd probably be able to beat people as strong as Nagato.