In my experience, wool does not "bag out.," if by that you mean stretch out. It might change form due to wrinkles etc, but otherwise no...In my experience (from long ago due to avoidance), polyester does not breathe and is hot as hell. I won't even wear polyester socks.
In the new Bergdorf catalogue I saw a melange green Kiton with a light blue overcheck, which was Wool/Linen/Silk/Cashmere.
Is there anything reason to do that other than saying "fuck it, lets just blend everything in it"?
Linen + cashmere sounds like a recipe for hot soft stiff cool wrinkles.