My teeny tiny burly is about 8x11, and I think I could push it to 12x12 no problem. The video Ryan posted of his primo CREAMING that plywood leads me to believe the claim that it’s more than a little improved in rigidity. Add those two facts together, and I think you’ll have no problem with the aluminum at your size.
About the z, my legs were so short that the printed parts touched. I don’t think that was holding me back, so I did add 1.5 inches to them a couple weeks ago so I could cut some molds and I haven’t had a chance to try the aluminum since then. Before that, I couldn’t cut anything that was an inch thick because my spoil board raised everything too high.
I still don’t have any noticeable flex in the legs, but I did cut some holes in the base, so they sit an inch lower than the feet. As long as your legs don’t flex when you put it together, you can always mount the aluminum to a block or something and raise it closer to the gantry so you don’t need the z sticking out the bottom so far.