Point is that Broda's and Durnan's GAA did not simply follow the league trend, they went down indicating a difference maker capacity. Brimsek's GAA followed the league trends.
Paul Bibeault was one of the NHL/high minor league straddlers.With Toronto in 1943-44 was a 2nd team AST behind Durnan. Replaced Durnan - injured, for 10 games with the Canadiens:
http://www.hockey-reference.com/players/b/bibeapa01.html
Your Broda replaced by Brimsek question is interesting. Let's extend it all around. Pre Red Line, switch Brimsek for Broda, Leafs/Boston trade and I see a wash in the final results - regular season and playoffs. Post Red Line. Broda / Durnan would be a wash. Brimsek / Durnan, Canadiens would not do as well. Bruins would do better. Durnan / Broda, would be a wash though the Leafs would have better regular season results. Brimsek / Broda, Leafs would likely win 3 straight SCs but would have more losses in the finals, upwards of 6 as opposed to 2. Bruins would be about the same during the regular season, maybe extending a series a game or two.
Based on your last paragraph, it sounds like...and this is my big conclusion: they were all pretty close...we're really making a ton of progress here
Well, like any constipated mathematician, I'll try to work it out with a pencil...
I think we'll find (well, maybe just you C1958, not everyone is especially entertained by our pre/post red-line banter) that Brimsek is more in line with his peers at home (where his sluggish defense could better protect him) as opposed to on the road, in sum. For gander taking: (sorry, I just don't know how to make pretty tables)
1946-47 through 1948-49 regular season
Brimsek:
Home: 45-23-17 - 2.49 GAA
Broda:
Home: 54-19-17 - 2.41 GAA
Durnan:
Home: 51-27-12 - 2.11 GAA
Then the friendly confines are vacated...
Brimsek:
Road: 30-44-15 - 3.12 GAA
Broda:
Road: 31-40-19 - 2.88 GAA
Durnan:
Road: 31-41-17 - 2.71 GAA
Durnan and the Habs were freakishly good at home, so that skews it a tiny bit. Given the information that we have though, I don't think that Brimsek just plumb forgot how to play on the road...there are other factors involved. To what degree they matter is a matter for the courts.
EDIT:
Of note, of varying significance.
1947 Broda @ Boston: 0-4-2
1948 Broda @ Boston: 1-3-2
1949 Broda @ Boston: 3-3-0
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1947-49 Broda @ Boston: 4-10-4
1947 Durnan @ Boston: 5-0-1
1948 Durnan @ Boston: 1-3-2*
1949 Durnan @ Boston: 3-3-0
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1947-49 Durnan @ Boston: 9-6-3
* - McNeil actually played Boston to a 2-2 tie in his only start of the season in Beantown (included as Durnan for illustration)
I'm not sure exactly what that means, if anything, but I was curious to see what it looked like anyhow...