I just got a couple of e-mails from Mike at GIK.
He had a number of suggestions I'm probably going to incorporate.
For the ceiling, he's recommending 4 x 4" thick panels, vs my original 3 x 2" thick. He's recommending 6" thick Impression series bass traps and diffusion for the rear, along with similar for the front.
Interestingly, he's kind of replacing lack of surface area with thicker panels. Mike also recommended standing Impression series panels in the front under the windows. I have been desperately trying to avoid blocking any windows, so these are the units I'm still unsure about.
While packing this past weekend I had to remove my beloved 2" thick panels from around the room, and I have to say that how obvious the difference was in the room. Not just was the room more sibilant but it practically buzzed.