Thank you all for your replies. I forgot to mention, I am a 5'1", 120 lb. woman, so a 30 lb. bike with another 30-50 lbs. of gear is relatively a bigger concern to me than a bigger rider (I think). So a savings of ~ 8-10 lbs. in bike alone is substantial.
If you read the bike description http://georgenaterry.com/coto-donana-tour/
it says it's designed for loaded touring
So does the lighter weight alone make it less stable under load (shimmy)?
RussSeaton, I'll look into other custom frame makers as you suggested, but I think they are all in this $3500 range , from some cursory investigation I have done. I liked this bike, but think I've convinced myself I prefer 26" wheels for touring.
You would not be saving 8-10lb. My Trek 520 weighs 24 lb. At 21 lb., you'll save 3 lb. With a load, I doubt you'll note much difference. A full waterbottle weighs 2lb. Think you'll go faster after you've drained a waterbottle?
Yes, you can have shimmy with a loaded lightweight bike. However, I'd trust Terry that this bike, although lightweight, is designed for fully loaded touring.
I'd investigate Co-motion, highly respected proven model at same price, with advantage of being semi-custom fitted to you.