To me, the immediate example of a QB unable to succeed despite having great pieces around him is Brock Osweiler. Granted, the Texans won games despite him for a variety of fairly flukey reasons, but...
Then you can look at a guy like Brian Hoyer, or Ryan Fitz. These guys go hot and cold, and that can work in the reg season, but they're not going to win in the playoffs, when they'll be counted on to bring their best consistently.
Peyton Manning's last year might be a good prototype for the absolute minimum QB contribution a team can receive while achieving ultimate success. Especially looking at the playoffs, it says to me that a team that is near the top of the league in multiple other important categories can win with a physical abomination at quarterback, as long as that player has the head to put the offense in good positions, to take only what is given, and to show mental discipline in the face of adversity. Basically, don't turn it over, make the most important throw, and don't get greedy, ever.
Anyway, I'd say regular season success and postseason success are vastly different categories, so it's tough to pick a single group. There's definitely a wider selection who can succeed with those superior assets in the regular season. I think it might be the baseline for a good backup versus a backup who needs to be replaced.
As to the last part of the question... It's hard to a imagine a great veteran QB accepting the coaching and scheme of last year. It's almost impossible to imagine what a Manning, a Brett Favre, etc. would have done in that system. Could we see the system actually work, when powered by a superior arm or understanding of the game? That would mean opponents respect the deep ball, give the runner room, don't pick up all the underneath routes... Perhaps... My suspicion would be we'd end up around 6 to 8 wins with a fired OC, a beat-up star QB, and a (still) frustrated Todd Gurley.