A hard shell made with a porous membrane, such as eVent, provides “worst case rain protection” in combination with air permeability as high as .53 CFM. This is what I measured on the Integral Designs Thru Hiker. I measured the air permeability of the Rab Boreas windshirt, which Dave C. gave the highest rating in his recent SOTM report; at 68.69 CFM but with no external moisture protection. THE DIFFERENCE is 68.69 / .53 = 1,296%.
To put the windshirt air permeability spectrum in perspective, the most breathable windshirt I have measured is the Acteryx Squamish at 100.59 CFM. The windshirt that will achieve the optimal balance point, for most people while UL backpacking, is the Houdini.
I measured the older Pertex Classic fabric and it was 3 CFM. Assuming this is the Pertex fabric used in your windshirt, it is a very poor choice for use while UL backpacking. I also measured the North Face Verto windshirt using the newest Pertex GL fabric and it was even worse at .53 CFM, the same as the Thru Hiker eVent. I then had a forum member, who purchases a lot of Pertex fabric, contact his rep. The rep's spec for the fabric was less than 1.97 CFM.
At a 7 MET exertion level (UL backpacking average), in average summer mountain temperatures and wind conditions, most people find the Houdini air permeability the optimal windshirt available. I measured the spring 2012 version of this windshirt at 35.8 CFM. The reason its air permeability is optimal is that this is the level of air permeability that will JUST PASS the AVERAGE EVAPORATED SWEAT moisture while UL backpacking. More air permeability than provided by the Houdini is not a wise choice for most people.
As air permeability goes up, wind resistance and external moisture protection goes down. More air permeability than is required to pass through your sweat is not a benefit. The key to selecting optimal windshirt air permeability is matching the air permeability that will just pass your average evaporated sweat level without venting. Venting can then be used for augmentation.
A windshirt that would never limit your maximum possible work potential requires ~400 CFM air permeability and would be worthless for protecting you from any wind or moisture. It is a balancing act.