He can play 5 and everything about him screams 5, except for his height and or vertical leap. Classic traditional post player that thrives on contact and scores usually in blue collar man fashion.
Also in terms of guarding, it would be much easier for him to be against centres than pf's like kg. Just too fast
If BBD did a great job playing C and was maybe a couple of inches of wingspan, reach or vertical jump away from being a c that could hold his own, you sort of have more hope for JS .
Sullinger has a good inch in height advantage , 1.5 to 2 inches longer wingspan and just as strong as bbd.
Not too sure how much more speed he can gain to be a starting pf in the league, but its either that, or tone up more now, build more muscles and try as a C or the worse we can expect just progress as he is now and be a backup pf/c for the future.