Regular season you are correct but when Marner splits from Matthews, he takes on the tougher comp allowing Matthews to feed. In all three scenarios, the lines are above 60% goal differential due to defensive acumen. The playoffs has been a bit different.
Over the last 3 playoffs 5v5:
Marner with Matthews:
Goals for/60: 3.53
Goals Against/60: 2.06
Marner without Matthews:
Goals for/60: 2.39
Goals Against/60: 1.18
Matthews without Marner:
Goals for/60: 1.66
Goals Against/60: 2.06
Both Marner and especially Matthews drop quite a bit offensiely when split up in the playoffs although Marner has an insanely low goals against. The main argument in keeping Matthews and Marner together is that Matthews has been pretty brutal away from Marner and they are a rock solid first line 5v5 in the playoffs. The whole team has blown chunks on the PP though.
I agree and said as much earlier.
... ignoring last playoffs, the previous 3 years Matthews and Marner with and without eachother.
2020-2023
330 mins with Marner
106 mins without Marner
Corsi Marner without Matthews: 42%
Corsi Matthews without Marner: 46%
Corsi Marner without Matthews: 44%
Corsi Matthews without Marner: 49%
Shots for Matthews without Marner: 49%
Shots for Marner without Matthews: 44%
xgf% for Matthews without Marner: 52%
Xgf% for Marner without Matthews: 43%
Scoring chances for Matthews without Marner: 45%
Scoring chances for Marner without Matthews: 40%
High danger scoring chances for Matthews without Marner: 53%
High danger scoring chances for Matthews without Marner: 46%
So... Marner despite playing with 11 million #1 center Tavares, without Matthews he's quite comfortably under 50% vs his opponents.
While Matthews, without an 11 million dollar linemate is usually pretty comfortably above 50%.
Looks like Matthews is maybe worth his money but Marner definitely isnt.