After looking tirelessly for 10 months I finally gave up on the SAUM idea and rebarreled my 7wsm again in the same 7wsm. There are a bunch of guys using the SAUM in tactical matches here in Sacramento, so I had hoped to borrow some well proven recipes but alas, a rifle is no good without components, so I'm sticking with the 7wsm.
I've ran into a good bit of 300SAUM brass (Nosler) lately. Yes you can neck it down, it shouldn't be a problem. Worst case scenario is you have to turn the necks, not the end of the world, just a PITA.