Stamkos seems to be the weak link for the Pens' duo line. Every other line has all 3 players who were, or are, playing at MVP caliber seasons. Stamkos hasn't ever really had a Hart/Lindsay type season.
Stamkos has at least one better season than Bure, Matthews, Datsyuk and Mackinnon.
His peak from 09 to 13 is probably better than all of those players four season peak too.
Richard 2x (2x runner up)
As-2 2x (Malkin, Sedin)
Point finishes 5, 5, 2, 2 (Stlouis 2x, Sedin, Sedin2x, Backstrom2x, Perry, Ovi, Crosby, malkin)
Stats over that period: 294-185-155-340
Lead league in points, goals, gpg (.62) and 3rd in PPG (1.16)
Considering Datsyuks peak is 05-09
As-2 (Malkin)
2x selke
Hart 3 and 9
2x #4 in point finishes (8th and 5th PPG) (Beaten out by Iggy, Malkin 2x, Ovi 2x, Crosby)
397-145-293-438
Datsyuk was 22nd in goals, 5th in points, 12th PPG (1.10)
Considering Mackinnons peak 2018-2021
AS-2 2x (McDavid, Draisaitl)
Hard 2, 2, 6
Goals 6, 9, 9
Point finishes 5, 5, 6, 7
Stats over that period: 263-132-209-341
3rd in points (Drai, McD)
6th in goals
3rd in PPG (1.3) McD, Kuch
Bure doesn't have a clear four year peak and doesn't hang with the others, we would have to consider a peak of two seasons.
Matthews isn't even necessary here, he hasn't hit his prime yet and he's the clear weakest link.
With all that considered, Stamkos is closer to Sakic, Kane, Fedorov than he is close to Datsyuk and Mackinnon.