SRS

Hockey Reference has a metric called the Simple Rating System (SRS) that is designed to measure team strength in terms of predicted goal differential per game. I haven't yet run the complete numbers to see exactly how accurate it is in terms of predicting playoff results, but I do know from hacking around a bit with the results that teams that have a large edge in SRS over their opponents are much more likely to win their playoff matchups.

It appears that the threshold where victory becomes highly likely for the favoured team is when the SRS gap between competing teams is .3 or more. Since SRS claims to be a measure of a team's per-game goal differential above average, that is the equivalent of a difference in goal differential of about +25, which equates to a difference of about 4 wins over 82 games in terms of "true talent" (i.e. the rating is designed to adjust for schedule strength and to remove some of the effects of variance over the course of a single season, such as a team that wins a lot of shootouts or close games).

Even the best goalies in recent times built their playoff records largely on series victories where their team had an SRS rating that was .3 or more ahead of the opposition. Here are playoff series win/loss records for the consensus top 4 goalies of the last two decades split by the strength of their teammates and opponents:

Patrick Roy:
On teams >.3 better than the opposition: 21-2
On teams >.3 worse than the opposition: 2-3
Playoff series against all other opponents: 10-8

Martin Brodeur:
On teams >.3 better than the opposition: 15-2
On teams >.3 worse than the opposition: 0-1
Playoff series against all other opponents: 5-9

Ed Belfour:
On teams >.3 better than the opposition: 12-2
On teams >.3 worse than the opposition: 1-4
Playoff series against all other opposition: 6-6

Dominik Hasek:
On teams >.3 better than the opposition: 5-0
On teams >.3 worse than the opposition: 0-3
Playoff series against all other opposition: 7-4

Combined results:
On teams >.3 better than the opposition: 53-6
On teams >.3 worse than the opposition: 3-11
Playoff series against all other opposition: 28-27

Playing on a much superior team, these goalies combined to win 90% of the time. When playing on a much weaker team, they only managed to win 20% of the time. In all other series it was essentially a coin flip.

It should be noted that the SRS rating is based on regular season performance that includes the contribution of the goaltender themselves. Keep in mind that in some of these situations, the goalie's own play was largely responsible for lifting their team ahead of the opposition. For example, some of the teams had lower win thresholds than their opponents yet higher SRS ratings, which would be an indicator of quality goaltending.

Also, just because the goalie is playing on the better team does not mean they weren't playing well, or were only overseeing 8-0 blowouts. It is important to avoid the common bias in favour of goalies on great teams, but we shouldn't overcompensate by never giving any credit to any goalie with decent teammates. Upsets can easily happen in seven game playoff series. A good team with a good goalie in net becomes tough to beat, as even when the team is off its game the goaltender can sometimes come to its rescue.

As a point of comparison, Chris Osgood was 12-5 in series where his team had a significant advantage. I'm not saying Osgood was to blame for all of those losses, but poor goaltending, or perhaps an average goalie who goes on an unlucky streak, can sometimes sink even a very good team. The high winning percentage in the mismatched series above does seem to indicate strong goaltending, although certainly the rest of the team played a huge role as well.

As an footnote, here are the numbers for Curtis Joseph:
On teams >.3 better than the opposition: 2-3
On teams >.3 worse than the opposition: 2-6
Playoff series against all other opposition: 7-4

In the latter two categories, Joseph's record stacks up against any of the other guys. The main difference was that he did not have nearly as many chances to pound weaker opponents as Roy, Brodeur and Belfour. Cujo was also quite unlucky in terms of goal support, as the Red Wings' failure to score on Giguere in '03 and Kiprusoff in '04 ended up being the main reason for Detroit's downfall in two of his three losses against weaker opposition.