I think making the first line and the shut down line mutually exclusive is incorrect. While I do not see Gus being a top ten scoring center, I do, like others have said, see him being able to score between 50-60 points per year.
On top of that, I think he's going to be one of the leagues top defensive centers, with the ability to play and succeed in all situations.
He will wear a letter for the duration of his Sabres career too, with next year being the latest this happens. I think it's 50/50 that he wears one this year. I think this year starts with Stafford, Gorges, and Gionta wearing letters. If Stafford is traded at the deadline, Gus gets his letter. If not, he starts next year with one.