In 1975 Bobby Clarke was on the ice for 19 ES (actually non PP goals) goals against in 80 games. This is during a high scoring era much like the early 90s where the average game saw 6.88 goals. In an entire NHL season just 19 goals were scored against the Flyers at ES when he was on the ice and 88 were scored when everyone else was on the ice. That's .24 ES goals against per game or roughly 1 goal against every 4 games. The rest of the team allowed 1.10 ES goals against per game. Clarke was on the ice for just 17.7% of the ES goals against while playing upwards of half the game. I don't know if people can truly wrap their head around how incredible that is or the fact that the following year he essentially replicated those numbers.
Just to compare, Bergeron's best season would have been 2012 when he was on the ice for 35 goals against at ES while the rest of the Bruins that season allowed 121 goals against. That season the average goals per game was 5.48 which is well over a full goal less per game than Clarke's 1975 season. Bergeron was on the ice for .43 goals against per game while the rest of the team allowed 1.48 goals against per game. Bergeron was on the ice for 22.4% of the ES goals against.
Clarke played more minutes, allowed fewer goals against in a higher scoring era, a lesser percentage of goals against, and still managed to put up 116 points to Bergeron's 64.
This isn't to say Bergeron is overrated by any means. Bergeron is an absolute legendary two-way player but this just goes to show how simply incredible Clarke was.
This is also better than anything Gainey ever did as well. Gainey was an incredible defensive player and the fact that the Selke was basically made for him doesn't make him the best defensive forward, it just means they wants to create an award to highlight players who weren't already being recognized and receiving awards. If the Selke was around in the early 70's, Clarke for sure would have won at least 3 more and that's being conservative.