Tried to delve down into per-minute stats for the guys that Tavares gets credit for boosting their scoring. This still is far from a perfect analysis, but it's better than the PPG stuff posted above.
Going to look only at Even Strength scoring here.
Parenteau
Age 27 NYI: 1.64p/60, 1.23p1/60
Age 28 NYI: 1.96p/60, 1.52p1/60
Age 29 COL: 2.13p/60, 1.74p1/60
Age 30 COL: 2.03p/60, 1.48p1/60
Don't think we can give any credit to Tavares for Parenteau, since he was more productive at even strength both years in Colorado than he was in his 2 years in NYI.
Okposo
Age 26 NYI: 2.03p/60, 1.57p1/60
Age 27 NYI: 1.86p/60, 1.45p1/60
Age 28 BUF: 1.28p/60, 1.09p1/60
Age 29 BUF: 1.28p/60, 0.97p1/60
well, there's very clear massive difference. Maybe Okposo just gave up on hockey after getting the big contract, but this looks like a pretty good example of a player's production falling off a cliff without Tavares.
Moulson
Age 28 NYI: 1.95p/60, 1.53p1/60
Age 29 NYI: 2.22p/60, 1.73p1/60
Age 30 MIX: 1.66p/60, 1.44p1/60 (half the season with BUF, quarter seasons with NYI and MIN)
Age 31 BUF: 1.58p/60, 1.20p1/60
Age 32 BUF: 0.98p/60, 0.75p1/60
Not as extreme as the Okposo example, but still a good example of Tavares likely boosting his production significantly.
other examples like Bailey and Lee we'll have a better idea of after this season, but right now we don't really have a point to compare on. I tried to look into the Vanek example, but i couldn't get a split of just his short time in NYI - and really, looking at his production with other teams that same year, I doubt it would show much Tavares boost - Vanek was really, really productive long before he played with Vanek.
But the Moulson and Okposo examples seem legit to me.