Who's having a sophomore slump? Paajarvi, maybe Omark, Gabner, Bickell, and maybe Tedenby. Ennis' point totals are gonna be a lot less but that's because of how bad BUffalo was at the beginning and his injuries...he's been their best or 2nd best forward lately.
Not really a lot there especially because Omark/Paajarvi/Bickell/Tedenby didn't do anything special last season to begin with. And Omark/Bickell aren't exactly good hockey players.
Which players have had a 4th season breakout is what I'd be more interested in. Maybe Simmonds to some extent and you can say Giroux but he had his breakout last year. Maybe Perron if you exclude last season when he was injured.
Kulemin is a major exception to this rule, serious regression in year 4. However, I definitely think there's something to this, most forwards do seem to break out around their 4th season in the league.
Interestingly, most rookies (those aged 18-21 when they enter the league) must sign 3 year entry-level contracts. If most forwards break out in year 4, then these forwards have not yet broken out when they become RFAs, so they don't get paid that highly. If entry-level contracts were 4 years, maybe we'd see higher RFA salaries? 3 years seems like the ideal entry-level contract length from the POV of the owners - any shorter and you're not maximizing the amount of time you get the player for cheap, but any longer and you have to pay more for a player who has really broken out. There's potentially some very smart thinking by the owners here!