I don't think his ceiling is a middle 6 guy. I think quite the opposite. I am going to quote a post I wrote earlier in this thread about his ceiling in a response to a similar question.
"I feel like you think that Stillman doesn't have as high of a ceiling as some of the other guys because his floor as a 3rd liner might be higher than some other "swing for the fences" types we could have drafted, and I would like to say that that is not the case at all. In terms of needs for the Devils, if Stillman hits his ceiling or really just develops like he is supposed to, he is exactly the kind of player you have on championship-winning teams. Stillman ceiling is a 30-30-60 first-line physically dominant power forward who is a beast in front of the net on the powerplay and surprises other teams with his impressive transition play, skating, and hands, and is the kind of player you throw out there with a minute left no matter if you are winning or losing. Obviously, I could go bonkers and say he can put up 80 points a year, but if we are being realistic, and absolutely everything goes right for Stillman, then this is the kind of player you are looking at."