I rode GDMBR last summer (most of it) and it is the Most Demanding environment! I was rained on for 30 days straight!

It was common to climb into a rain - no surprise, mountain passes attract storms - and freeze on the way down. Let's not forget, riders get up in the morning, pack it up, climb like hell and summit when the afternoon storms are getting good, when the only desire is to get the hell off that summit. It was common to encounter ice storms on the lee-side of the passes. The other major problem involved climbing (you want lots of vents) with descending (you freeze) with camping (wish you had another warm layer).

I went with an REI rainsuit and survived.
I'm glad I didn't destroy a $200 rain jacket. Save that for Seattle roadies.
I don't think a wool sweater will cut it. Every now and then you'll just have to "go to ground" because the rains are so bad, and the road turns to sticky soup!
Your conditions go from freezing hailstorms to 90-degree floods to hiking up a river pushing a loaded touring rig. You can't really win this battle - instead, focus on change of clothes that dry quickly.

I learned to love laundromats - everything was wet all the time, it seems! That funky smell - ack -