One will practically always need to EQ the low end from a sealed sub to resemble anything flat, so I don't see much use in getting caught up with the natural Q of the system. A larger sealed enclosure will get you a flatter down low with less EQ and a little less low end distortion due to less cone resistance and potentially less thermal compression, but if these are major concerns, one wouldn't chose to go sealed in the first place, as lack of flatness down low and high distortion are inevitable with such an alignment. It's a trade of enclosure volume vs power. So in short, yeah, might as well go small and EQ it flat for something like this.
I'm not sure, but I think you can get pretty big discounts from Mach5 if you order multiple drivers. That might be the way to go if you were interested in the stacking floor to ceiling idea.