STL+BLK on a per 100 basis is average, from what I see (for wings)......but he is kind of right on that borderline of good defensive potential, from a DBPM, DWS and DRTG perspective. Considering how much water he carries on offense, it's actually very uncommon that a wing has even average defensive metrics on defense.
It's a little murkier than you would like......and I'd really have to dig in on lineup combination data, as I have no clue how good any of his teammates are but the markers are there for him to be an undervalued player......certainly based on past data for wing players. Guys who can truly play the SF position almost never creep in to plus territory in my stuff at the college level.....and the handful of guys that do have a really high rate of success at the NBA level.
I'm still kind of on the fence with him.....but he does stand out analytically. Of the 10 wings in his rank baseline or above (only 15% in the database), 8 were at worst, plus VORP players at the NBA level. He just sits (currently) in this sweet spot that few wings get in to (1.1+ in overall impact).....and especially scoring wings tend to translate those skills when their impact is this positive at the college level.