One of our steel match guys (and old 1k shooter) took a stock savage 308 to the IBS nationals with his can on and shot two back to back 5-6 inch groups for 5 shots at 1000. which by the way placed him in the middle of the pack at least.
Pretty sure a suppressor does not hinder in any way.
About half the guys shooting a local steel match out to 900 shoot cans and trust me, the suppressors are not a detriment. I just ordered the new Liberty 30 cal magnum suppressor in titanium.
Right now only the Thunderbeast 30-P1, Liberty Vicory Magnum (not rated for the Rum) and a couple othes will take the magnum cartridtges. If I was going to the RUM and larger, get the 338 rated cans and in titanium. I would go threaded, and just buy an adapter IF the gun is already built and if building one have the smith thread the brake for the suppresor threading.