In order to answer this question, I made a list of defensemen over the last four seasons who averaged more than 22 minutes per game and played between 20 and 70 games in a given season. 22 minutes of TOI corresponds approximately to the 60th-best defenseman, which means that the group of subjects includes mostly 1st and 2nd defensemen.
I then counted up the number of even-strength goals and shots against when each defenseman was in the lineup and when he was not dressed. I further split the defensemen into those whose +/- was better than the team's overall +/- (on a per minute basis). Presumably the group with the "Better +/-" would have more of an impact on even-strength defensive zone play.
GP G/GP S+G/GP SVPCT GP G/GP S+G/GP SVPCT DELTA
N=28 Better +/- 1610 1.85 21.48 914.0 686 2.04 21.94 907.2 5.8
N=25 Worse +/- 1229 1.89 22.20 915.1 821 1.89 22.00 914.2 0.9
The assumption here is that during a given season, a player's aggregate group of teammates, including the goalie, will be basically unchanged whether he's in the lineup or not. Similarly, his team's aggregate group of opponents is assumed to be the same whether he's in the lineup or not.
While the defensive difference with the "Worse +/-" defensemen out of the lineup was negligible, the teams of the "Better +/-" defensemen allowed 15.5 fewer goals per 82 games when they were playing. One-quarter of this change was due to reducing the volume of shots allowed, but the rest is due to better save percentage.