In reply to whiting.jp:
I dont think that, within 3 minutes or 500m, what is worn underneath will have very much bearing on the situation at all, be it cotton, synthetic or merino. If it's getting wet that quickly, then it's going to be the outer layer that is leaking. Unless you're running in a sauna.
I've heard mixed reports of the ReadyMix's ability to see off rain, but, if the DWR is like that on the shell of my Stretch Zephyr, I'd expect it to last a lot longer than 3 minutes.