UPDATE: The new 2013 Patagonia Houdini has changed. The new fabric is not breathable at all – on par with the MHW Ghost Whisperer. This is a major disappointment and I can no longer recommend it as a windshirt for aerobic activities. With the Stoic Wraith discontinued. There are NO windshirts on the market that I'd recommend for aerobic activiites. You'd be better off saving the weight and just bringing a rain jacket.
A couple weeks ago, I ordered just about every windshell out there. I sat down and analyzed the breathability of each one.
North Face Verto
Montane Slipstream GL
Montane Lite-Speed
Montane Featherlite Smock
Patagonia Houdini
Stoic Wrath
Montbell Tachyon Anorak
Mammut Wing Half-Zip
Mountain Hardware Ghost Whisperer
Two fabrics were noticably more breathable than the others:
Stoic Wrath (proprietary)
Patagonia Houdini (proprietary)
These were all in the same class:
Montbell Tachyon Anorak (proprietary)
North Face Verto (pertex quantum)
Montane Slipstream GL (pertex quantum)
Montane Lite-Speed (pertex microlight)
Montane Featherlite Smock (pertex microlight)
These were the least breathable, but were just SLIGHTLY behind the ones above:
Mammut Wing Half-Zip (proprietary)
Mountain Hardware Ghost Whisperer (proprietary)
Nothing too scientific, but with the 'breath' test, it was pretty obvious which ones breathed more than the others. Just thought I'd share my results.
I still like my Montane Lite-Speed for climbing and canyoneering, since it's very durable, has an excellent hood, breathability isn't as important for non-aerobic sports (especially when on belay, or trying to solve a problem). But the winner for me when it comes to backpacking and peak-bagging was the Stoic Wrath – full zip, packs into a pocket, one of the top 3 lightest jackets, and very breathable. And… it's super cheap right now. While it doesn't have a bill, I always have a ballcap with me, so with that addition, the hood works well.
If I had to wager a guess, the Wrath and the Houdini are around 5 CFM, the others are about 1-2 CFM. With the Wrath and Houdini, I could comfortably breath through the fabric (with some resistance). For most of the others, it was quite difficult to breath through the fabric (gave a vague sense of suffication), but still possible. For the Mammut and MHW, it was near impossible.

