The main issue I see with what the Celtics did, is that people assumed that by letting Sullinger walk and adding Horford, we got a HUGE upgrade. In reality though, we added Horford, someone who could rebound slightly better than Sullinger, but also lost Sullinger, our BEST rebounder last season. (And I don't blame the C's at all for that. Even I thought it was okay letting Sullinger walk at the time, I'll admit)
So in essence, our rebounding really didn't improve at all, and sadly none of the other bigs are really helping consistently in that department (heck, even Bradley leads this team in rebounding...).
While Horford brings more to the table than Sullinger and overall has still been a great fit, the answer is YES, a healthy Sullinger would make this team better, and would help the rebounding a bit, although this team really needs someone who can terrorize the paint and rebound consistently (a Tristan Thompson or DeAndre Jordan type).
Hopefully Bogut or Nurkic is available at the right price.
What does surprise me though.. is that Sully went to Toronto for 1/6M.. but we're paying Zeller 8M/Year... we should have kept Sully not Zeller but I won't rant much about that. Sullinger could have also been that "Amir-like, or Zeller-like salary filler trade piece" as well.