This team is a lot of fun, but they can't be taken seriously without an upgrade in the middle. Look around the league -- EVERYBODY has better defensive bigs. There might be 3 teams with worse interior defense. It's not a winning formula. Horford isn't a world beater on defense, but he's very solid and a big upgrade over Olynyk. I think he'd pair very well with Sully and make one mean frontcourt.
There are some question marks around Horford. His health, and whether Atlanta will want to re-up after next year, and for what amount. It all hinges on his play and health this year. I say roll the dice. Bundle a first-rounder (or two!) with Jeff Green, Bass, Olynyk, cap relief... whatever. Basically anything outside of Rondo, Smart, Bradley, Sully that makes sense.
What would it take? Any Hawks fans out there?