I've come to the realization that in summer I'm getting wet weather it rains or not. I went backpacking this weekend. The first day there was no rain, or at least not enough to penetrate the tree canopy, and the second day there were storms and on and off rain all day. I didn't wear a rain jacket the day it rained, but I did wear my garbage bag rain skirt. I was equally wet both days.
In other words, in the summer you're getting wet no matter what, whether it's from sweat or rain. So why bother with the jacket? It's only going to make you hotter and unless you move at a very slow pace you're going to be soaked from sweat.
I will be carrying a jacket for in camp though. For that I have one of the sil jackets from Lukes. It has the pit zips which make a big difference. But it, and any jacket you get from him, needs to be seam sealed.
I agree with northermark, nothing is going to work like you want it to. That's just the nature of the beast. But if you decide to adopt the approach I'm taking then the pertex should work just fine.
"As a well spent day brings happy sleep, a well spent life brings happy death." -Da Vinci