I hike in the East - probably not as extreme wet as you get, but we do have our moments. I have hiked, both uphill and downhill, for 6 - 8 hours in a steady, heavy rain splashing along the small creek running down the trail. I've done this while wearing a poncho, a GoreTex rainsuit, and a Patagonia proprietary w/b rainsuit (different hikes, not all at the same time!) I did not stay dry: my T-shirt was probably 50% wet, my shorts were 30% wet, and my socks and shoes were 100% wet. The type of garment did not seem to make any difference at all in how wet I got. The moisture came from both inside (hot day, with sweating) and outside (hot or cold day, leaking in around the hood, mostly, plus running down - or up? - my arms since I was using hiking poles.)

As far as strategy: The only good way to deal with rain is to get out of it. Barring that, I would probably scrap a trip now if the forecast were for rain most of the trip. I'd still go if it was only a 50% chance, or if it were only going to rain, say, 1 day of a 3 or 4 day trip. I might modify my plan to make the predicted rain day a short day, or to spend the night at a shelter, to make things a little less difficult. (I used to do the whole non-pantywaist, "I've been wet before" thing and go regardless of the forecast. At 59, I no longer feel compelled to prove that I can.)