Now you are getting into hypothetical scenarios of "Put Howe on the 2009 Pens instead of Crosby" or "put the Pens up against the 1954/55 Leafs and Habs". I can easily counter with "Put Howe's linemates on Crosby's line". And listing a GAA is lacking in context on two fronts: it in no way reflects how that team played defensively in the actual playoffs and a raw GAA from one era should not be compared with a raw GAA from another straight up; they should be measured against their direct peers.
Here are the facts
The Pens, the #8 seed, played the #9, #4, #11, and #3 seeds in 2009 out of a 30 team league. The Wings, #1 seed, played the #2 and #3 seeds. The Wings were the defending champs, the Pens had to play the defending champs. In theory, the #2 seed in '55 would have been seeded in the #6 to #10 range, the #3 seed in ' 55 would have been seeded in the #11 to #15 range. An argument that the '55 Habs were more elite than the #6 team in '09 needs to be tempered with the fact the '55 Leafs may not have been a playoff team in '09, let alone close to a #11 seed.
From a seeding standpoint, it should be a wash in terms of the strength of teams they played; if anything it could be argued the Pens had to play two #1 seeds (including the era's best team) and two #3 seeds vs. the Wings playing a #2 and a #3.
As for actual defensive performance, the '55 Leafs had the highest GA in the playoffs, and the Habs had the 2nd best. The '09 Flyers had the 5th worst GA after round 1, the equivalent of being 2nd worst in '55, the '09 Caps had the 7th best GA after Round 2, the equivalent of being 2nd best in '55, the '09 Hurricanes had the 6th worst GA after Round 3, the equivalent of being 3rd best in '55, and the '09 Wings had the best GA after Round 4, the equivalent of being the best in '55.
Based on that, it seems to be a wash in terms of the relative defensive strengths based on actual playoff performances.
I can appreciate the narrative around Howe's '55 performance but from a strictly statistical standpoint, they are very close and cannot see why one would be on a tier above the other besides the expectation that Howe should have the unquestioned best playoff run his era given his regular season resume. It clearly takes some spin to try to establish this while for the other Big 3, it is clear as day that they were above their peers.
The narrative around Crosby's '09 performance is he, along with Malkin, carried the Pens to the Cup as a duo in a manner that is among the best of the last 30 years against the team that is viewed as the best of the post lockout era. I think you would be challenged to find a team that had two players, from different lines, that provided as much of their team's offense as they did.