Waryn,
I suspect that you'd regret not enlarging that floorplan. If you start with a minimum width of +/-425 and go from there, you'll probably be closer to the mark. You can probably get away with less flare to the side walls (smaller maximum width) if that is helpful.
BTW, if that's record racking behind the speakers, I'd also suggest that you stagger the depth of the racks, pulling some sections closer to the listener than others. IME, this is a great wall treatment. Similarly, you may want to put some racks (or decorative pilasters) flanking the doorway on the wall behind the listener.
Finally, while the ceiling arrangement looks good, you can also get away with drop panels of varying depth if that proves easier/cheaper.
Good luck
Marty
PS If you haven't purchased the subwoofers yet, you may wish to consider this approach:
http://www.audiokinesis.com/product_ak_swarm.html
I'd give Duke (the manufacturer of this system) a call to discuss. He's a very good guy who participates here regularly.