I honestly struggle with this.
Right now he's a very valuable two-way winger, who is pacing for high ~22g, 16a over 82 games. His raw 5v5 ice time has been that of a top-line winger, and his most common forwards are TK (~40%) and Couts (36%). But, he's played meaningful minutes with other players in non-offensive roles as well. So, while the raw 5v5 ice time is indicative of top-line winger, it's a bit more nuanced and somewhat explains the less than stellar point production.
I haven't seen a lot of Foerster prior to this NHL season, but I've seen enough to know there's still more to his game that has yet to emerge. For example, there's a subtle passing game in there that hasn't been exhibited at the NHL level with any consistency. In general, I feel he needs to quicken up all parts of his game to take another step forward.
In terms of overall production/total upside, I'm not really sure -- I really don't know. He's already proven to be a VERY good middle-6 NHL forward, which is a huge win at where he was drafted. I'm pretty confident that his assist total is low, and will likely increase next year. I wouldn't be shocked to see Foerster turn into a 22g/23a/45p player, but I don't know if I see much more than that. I just don't know if he has enough skill to be that offensively-dominant player.