The footprint is only 10 sq ft bigger than the SL5. That's less than 1 sleeping bag. It just looks a lot bigger because it goes straight up instead of sloping. It's a lot more usable space, though, with the sidewalls and extra shoulder room. You can put a sleeping bag almost against the wall without getting wet.
That's a big deficiency with the tipi. The outer 1.5-2 feet is pretty much unusable space for anything other than small gear so the footprint dimensions are deceiving. The added sidewall in your design makes the full footprint useable. Like you said, you can put a sleeping bag almost against the wall. Move it out a bit and you can sit up in it!
I'm just impressed as heck at the whole thing. Can't wait to hear a "trip report" in it.
