I see max as guys like Durant. Barnes is a little more like Tobias and Kris Middleton or something
I'm a bit iffy on whether Barnes is worthy of a max contract, but I do see him as being a significant step above Harris and Middleton.
Main reason is that Barnes is the only one of those three guys who combines elite physical gifts (length, height, athelticism) with excellent shooting (40% from three last year), versatile scoring and quality defense.
Middleton has the shooting and the defense, but lacks the athleticism and the ability to create his own shot.
Harris has the physicals and the scorign ability, but isn't an outstanding shooter or defender.
The other big positive is see in Barns is that he has played last year on a team that also contained Curry, Thompson, Green and Iggy and yet he was STILL able to put up strong offensive production. There is no telling how much his offensive production could spike if he were to move to a team where he could be a #1 or #2 option.
By comparison Harris put up his numbers while playing big minutes and a big role on a bad lottery team, and Middleton put up his numbers in a pretty large role on a fringe playoff team. This tells me that neither of those guys is likely to see their production jump rapidly if they moved to another team - what you see is probably what you get.
That said Barnes is a big risk for the same reason - you'd be giving him a max on the hope that he does break out unexpectedly.
I'd probably be willing to take the gamble of there aren't any better propects available. He's a huge upgrade at a position of much need (SF), and he's a potential two-way impact player from the get go, and he's young enough to grow with the rest of our roster.