I have a Horizon the same dimensions - with the second tier as well. I cooked 16 slabs of baby backs today - no problem - just a little shifting mid cook. Cooked 8 butts on thursday with room to spare. The issue is how much your time is worth. You're gonna babysit that thing. Not exactly "set it and forget it."
If you want to see how big the Horizon is go to youtube and search for my videos - monty3777.
I shot a video the day I brought mine home.