Technically that shouldn't work, S setting bleeds off more gas to compensate for the extra backpressure from the suppressor. 300BLK should work fine on the U setting if your gas port is correctly sized. I've never switched mine to S and I've run three different non restricted 223 barrels, my 300BLK barrel and the 6.8SPC barrel I sold. All ran fine on U.
Meh, it is an impressive round but it all depends what you want to do with your rifle. I already have a good hunting rifle that beats the hell out of the 30AR and I already reload for 300BLK so even though I wouldn't mind a 30AR conversion I wouldn't give up my 300BLK for it. I can still hunt with 300BLK as long as I keep it to 150 yards or less in order to ensure a humane kill on a deer.
If someone isn't a handloader and actually wants to shoot their rifle on a regular basis outside of hunting season then the harder to find and more expensive 30AR isn't so great any more.
Not everyone needs the most powerful flattest shooting round available for everyday shooting.