Floorless design has some flaws, in case you haven't thought of them yet:
1. Rain. Slope is hard to predict on flat ground. If you get some rain, you'll get water come in somehow from underneath your walls. An insulator sleep pad will keep part of you out of the water, but your sleeping bag will still make at least moderate contact. This will reduce comfort and also drop body temperature.
2. Bugs/wildlife. An enclosed tent is a wonderful thing, and in many parts of the country... not just the poisonous southwest. Problems I've avoided here include black widows, tarantulas, scorpions, centipedes, ticks and other creepycrawlies. I've camped on South Padre Island in Texas near Corpus Christi, and had little blue crabs running sideways at my tent and paw/crawl at the walls of it for 30 second to 1 minute bursts, then rest for a few moments and try again. Creepy. I remember earlier in the night playing a game with the crabs in the dark. It was like that Vin Diesel movie, Pitch Black. I'd turn on the flashlight and aim at a spot on the beach and the crabs would run away from the circle of light, just on the edge of it. They'd close in behind me and I'd point the light over there. There were THOUSANDS of them on the beach. If I laid down on that sand without cover, I would have been pinched by hundreds of them and they wouldn't have left me alone until the mass of them either drove me off the beach, or literally tore me to little pieces. I guarantee that those buggers would find a way into a tipi.
3. Snakes LOVE residual night warmth from stones that have been warmed by fire. You have a floorless tent with a stove inside. 'Nuf said.
Keep those things in consideration if you do use a floorless tent/tipi. I've done rough-it tarp and tentless backpacking in parts of the country that have less reputation for creepycrawlies and snakes so it certainly is possible and convenient to save on the weight... but I'd rather have a couple of cheap WalMart $20 two man pup tents than a tipi for bugout or long range backpacking trips.