I've had 2 of these uppers and both ran just fine with subsonic and supersonic loads right out of the box. I never went the adjustable gas block route. I will say that with the pistol-length gas system and whatever size gas port they are using (I've never miked it), recoil is definitely noticeable with supersonic loads. I'm pretty sure they set them up to make sure they will function with subsonic.
I still have the second one. I had the barrel taken down to 10.5 inches for use on a Registered SBR lower I have. Even at 10.5 inches, it runs like a champ with subsonic or supersonic loads with or without the suppressor. Here's a pic of the shorty.
