That's what I thought--300winmag is really referring to spinnaker fabric.

I like it--my Gossamer Gear/Tarptent Squall Classic is made of it. With a good taut pitch, the reputed noise (which is evident when setting up and taking down the tent) is not a problem. I did find it sagging when it rained, though, but that appears to be mostly the guylines rather than the tent fabric.

Considering that most silnylon tents (such as Tarptents) have guyline tighteners that can easily be reached out the front door without getting wet, I haven't found silnylon to be a problem, either. The main thing is to tighten everything up really snug just before you go to bed.