I did this comparison over the summer based on his rookie stats, so I'll update it here.
Here is a list of every player in NBA history who, at age 22 or under, averaged better than 18/9/2 per 36 minutes (and also played serious minutes):
Duncan
Barkley
Blake Griffin
Bosh
Kevin Love
Larry Johnson
Chris Webber
Clark Kellogg
Elton Brand
Kevin Garnett
Antoine Walker
Demarcus Cousins
Terry Cummings
Cliff Robinson
Otis Thorpe
That's it. Quite a select group.
If you go and pin down the group whose stats are most comparable, and also focus on guys who were mid-range shooters more than post players, you have:
Bosh
LJ
Cummings
Kellogg
If I had to generalize I would put all of those guys in the "occasional All-Star" category.
Now, the one thing that really differentiates Sully is his 3pt shooting this year. If you extend the criteria to include ">1 3pt FG per game" then there are literally no matches. Of course, for the older guys this is no surprise given how the game's changed. But it's worth noting that Sully's closest match in the modern era - Bosh - has never shot more than 2.3 3 pointers per 36 minutes.
Bosh's career percentage is lower than Sully's this year, and I think it's reasonable to expect Sully to improve at least a bit.
If you project Sully to improve his shooting while maintaining a decent volume, he becomes really unique. Not Kevin Love unique (due mainly to rebounding), but an inside/outside presence of very high caliber.
Here's a link if you want to play around with the criteria.
http://bkref.com/tiny/r1q3X