Sully- Horace Grant/David West
Olynyk I cannot tell
It would take quite a transformation for Sully to become the force that David West is. Sully has shown exceptional touch around the basket and has potential to be a great post player, but I don't think he'll ever be near the top of the league in the pick and roll/pop as well like West. David West gets by with his lack of explosion due to his elite fundamentals (Duncan-like) and vast array of post moves, neither of which Sully has really shown. While he should be at least as good as West as a rebounder, he hasn't shown the potential to become close in passing or overall defense.
More realistically I think Sullinger at his peak could be David West or Boozer/Zandolph as someone else suggested, AKA 20/8-10 for a year or two, but not consistently year after year throughout his prime like the other 3. I doubt the rebounding will hold him back, but even if he can hone the same jumper as them, he hasn't shown the potential to be enough of a scorer to be the 2nd option on a contender (or 1st in Randolph's case).
He's going to have to work really hard to be a 15ppg player in a league of much more athletic players, 20ppg is a much tougher feat/higher ceiling. I think more realistic comparisons would be Scola, Blatche, Kaman, Haslem, or if not (like say his back troubles him) a Blair/Hickson/Big Baby type.
As for Olynyk, even though his ceiling is higher, I'm liking Boris Diaw as a probable outcome. They are much more talented than their peers but have such an athleticism disadvantage that they can't reach their ceiling.