I concur with the Z-Bo comparisons - that's probably a good body type to compare Sully with.
I think though, that Sully is likely to be at least a marginally _better_ rebounder than Randolph for a few reasons.
1) as mentioned, he's a little bit more athletic
2) One of Sully's advantageous attributes is his velcro hands. He rarely fails to catch passes into the post and rarely fumbles the ball away (Not that I'm mentioning any names of former Celtic big men ...). If the ball is near him, he is like glue. This applies to his rebounding as well.
3) Sully, offensively, is more likely than most to play near the post. This will put him in far better position to grab offensive rebounds than most Celtic big men the last couple of years (since Shaq, really). Even though Doc will continue to NOT emphasize ORBs, Sully will almost certainly, by virtue of positioning, grab quite a few.
4) Defensively, Sully is not highly mobile. I expect Doc to try to usually put him on the floor mainly with another big who has good lateral mobility (i.e., KG) so that Sully can focus on post and weak-side defense. This will leave Sully again, closer to the basket.
All these things, combined with his well-known superior lower body strength and footwork lead me to believe that Sully could end up with very good rebounding numbers.