There might’ve been a season in there in which Spahn was far better than the rest of the Braves, but, for the most part, the Braves finished in the top half of the NL during his career (1st, 2nd or 3rd every season from '53 to '60).
Not judging the OP, but winning percentage stats aren’t terribly illuminating. Too many variables and the obvious problems with “wins” as a statistic.
There are so many ways to measure dominant pitching. Adjusted ERA+, for example. When you look at it historically, the names aren’t a big surprise, but some of the numbers are. Everyone knows Bob Gibson was a monster in 1968, but that sometimes gets explained away as happening in a weak-hitting era. It’s somewhat astounding that his adjusted ERA+ was 258.
That’s true but the OP’s question was pretty clear and on its own it’s actually quite an interesting question.
Where it gets complicated is in how you calculate it. Just going by the team record during a pitcher’s seasons with that team can be a little deceiving.
Let me use Walter Johnson as my example; in 1907 Walter pitched for a truly horrible Senators team that went 49-102. In 1912 he pitched for a decent Senators team that went 91-61 (he was by far the best player in the club but they’d have been decent without him.) 49-102 is worse than 91-61 is good, so that’s a net negative, right?
Well, you might already have figured out the trap here; Johnson only pitched in 14 games in 1907, 110 innings. He pitched more than three times as much in 1912, so the good Senators team in 1912 has for more impact on his career than the terrible 1907 team. A reverse example exists with Nolan Ryan; the best team he ever played for were the 1969 Mets, but he only pitched 89 innings for that team. You can’t really assign equal importance to those teams in terms of how much better Johnson was.
For a pitcher with a REALLY long career this probably doesn’t have a huge impact, but still.
You’d think, but during Fergie’s best years with the Cubs (1967-1973) and the Rangers, they were actually decent teams. In fact, his record seems to show he had very good years with good teams, but sub .500 years with sub .500 teams.