Nash- My top player, Nuno Candeias, has 236 goals in 122 league games, and his first 30 games he was under 5 stars. I picked him up as a 3 star, and he didn't score that much. He is way over 2 goals per game the last handful of seasons.
He also did really well a season when he started as a low 6 star, but he completely stopped scoring at that level in the Association games. HFF is top ranked in my association, and it often draws a 130+ opponent. This year I got him back to 8 stars, and he has been sick in FA games again.
In the league, I don't think it matters a ton if a player is 7 stars or 6 stars or 8 stars when you mostly face 2-3 star opponents. Position is more important than anything.
I also think the amount of rest a player get also matters a ton. If you work a player hard constantly, they don't perform that well. But if they are left alone and don't play all games, they tend to get really hot. But, its just speculation on my part!