When calibrating with Matthews and Kuch it’s important to distinguish PP from 5v5.
Marner is a full tier below Matthews in production and in addition his line mates are better.
Careful at comparing total season points between Kuch and Marner because IIUC Tampa has had a shot ton more PP time than the Leafs last few years.
p60 stats and isolating PP from 5v5 will give a better baseline when just looking at production.
The problem I have when people purely use per 60 stats is it doesnt include the risk of injury which probably should have been looked at with Matthews but I do think it is an important stat to look at for sure.
I'm pretty sure Marner out produced Matthews in points/60 at ES or 5 on 5 this year but that is largely due to linemate quality as you've said.
Looking at last 3 years for Marner vs Kuch for current contracts (excluding current year for Kuch as he signed his current deal early)
Marner
2016/17: GP 77, 5 on 5 Points 37 (2.1/60)
2017/18: GP 82, 5 on 5 Points 36 (2.0/60)
2018/19: GP 82, 5 on 5 Points 58 (2.9/60)
Kucherov
2015/16: GP 77, 5 on 5 Points 33 (1.9/60)
2016/17: GP 74, 5 on 5 Points 43 (2.4/60)
2017/18: GP 80, 5 on 5 Points 59 (2.9/60)
So Kuch out produced Marner other than the first years I compared. Interestingly 2015/16 was a bad year for kuch and he produced 44 5 on 5 points (2.7/60) the previous year. Also a bit of a warning to see such a huge sudden jump for Marner when JT arrived while Kucherov had steady increase each year.
I just dont think Marner should be getting that much more than 9.5 x 8 especially when you consider UFA years bought. I think he will get more AAV and less term, it will be an overpayment.