I strongly disagree with the notion that a suppressed SBR will not benefit from an adjustable gas block.
I'm very fortunate to have been able to purchase a Colt M16A1 full auto lower a LONG time ago. But, I typically don't shoot full auto these days other than to check the cyclic rate of a given weapon with an electronic timer.
Based on my own experience, 715 - 775 rounds per minute rate of fire works well for most AR' format weapons, regardless of barrel length. Ammo type and recoil system format cause far more variation than I'd ever imagined, unsuppressed. Change the cyclic rate 100 RPM by going one step lighter buffer?!? As wild as that sounds, you can't imagine how surprised I was to see what happened suppressed.
Add a suppressor, and what do you get? Well, it depends somewhat on the brand and the model of the suppressor. And ammo! Surefire suppressors have lower back pressure than any suppressor of which I am aware. That's one of many reasons I only use them on gas guns. But, strap one on and still watch the cyclic rate jump up 35%!!! It is much more with some brands. I've seen some well liked brands choke an SBR AR' to death.
When an AR' runs faster like this, it is only the rearward motion of the bolt carrier group that is faster. The recoil spring pushing everything forward operates the same in semi or full auto. Just hope that magazine spring gets the next round in position in time!
So, what's the downside of this faster rearward BCG business? The biggest problem, I think, is the brass does not have time to relax fully prior to extraction, for starters. Plus, there's excess wear and tear on all associated components. And more felt recoil. Harder to keep your sights on the target shot to shot. Keep in mind this happens in semi, too, shooting suppressed, not just full auto!
My 10.5" shown above runs at 720 RPM with M855 suppressed, but slightly faster with my ammo of choice, ASYM 70 grain TSX. The apparent recoil is next to nothing, noticeably less than unsuppressed. The weight of the suppressor out front keeps the dot on the target. Less gas is getting dumped where the gun eats. And it is uncannily accurate! If I told you what it would do, you would not believe me, and I wouldn't blame you for that.