Luke,
I've used a floorless Virga tarptent in some pretty high winds and heavy rain. I was camped on a lake with not much cover and a storm blew in. I lowered the pole and pulled down the one side that was getting more rain and wind and things were just dandy. There was a bit of misting along one edge, but the ground under me was dry the entire time. I used a plastic ground cloth that was just a bit wider than the tent so it could go up over the edges to keep water out.

I don't know what style you're interested in, but Silnylon can take a beating. Now, if I were in that mess for days...it most likely would seep thru. But I'm guessing here.

Hope that helps.