Oh, Marino would provide a good improvement. In OTL, the Steelers had Mark Malone in the 80's, who was terrible. Even if he isn't competely unleashed, he still does way better than Malone did.The Steelers have always been at the very least a decent team since the merger. They've never gone more than 4 years between playoff wins and have only finished with less than 6 wins once since 1970. There's a reason that Chuck Noll lasted into the 90's.
Honestly you probably don't have very far into the future to see the effects of Marino going to the Steelers. Marino actually faced Pittsburgh in the AFC Championship game for the 1984 season, you better believe that the Steelers team is going to be a lot better with 5,000 yard Marino instead of Mark Malone under center. I still don't Pittsburgh gets past the buzzsaw of the 49ers, but that's still a vast performance of what the Steelers did OTL. But I guess there's a question of if Marino would have been allowed to be unleashed like he was with Don Shula. If Noll isn't willing to stray away from the traditional pound the rock culture of the team, then I'm not sure how large of an improvement Marino would provide