I think he's a significant step below Marner at the same age, though obviously time will tell on that one.I'd use Marner's season this season as a benchmark for Keller. If he can replicate that, he has to be in consideration.
I also don't think that Marner would make Team USA in his rookie season (if he were American and the Olympics were this year). With that being said, JvR likely would make the team, and I do think Marner is already better than JvR, but for the Olympics there's a strong preference for experienced players, it's very hard to make one of the top Olympic teams as an NHL rookie. Also, the competition at RW is very strong, with Kane/Kessel/Wheeler the likely top 3, and you wouldn't pick someone like Marner for the 4th line. At LW the competition is slightly weaker, but even still, I really don't see rookie Keller making it.