How do you know he has additional pressure? Suppressed AR's run dirty, period. I suppose you could adjust the gas down to a minimum and I hear enough of you claim they work to doubt it, but it WILL mess with reliability --if you are getting almost no extra gas using a suppressor, how do you expect it to run right when you take the can off? Adjustable gas blocks aren't necessary, Eugene Stoner never needed 'em, the military never needed 'em and I don't need 'em. I do need my rifle to work regardless of ammo or suppressor though, and since most factory rifles are designed that way, that's how I build mine. I use the correct buffer weight, I don't make shit up or play with different weights. I look up the chart and get the buffer required for the barrel length and caliber. IF I need a bit more umph, I use a Wolf+ buffer spring; only needed to do that once or twice.
IF I got an adj. gas block, the only one I'd consider is the Noveske switchblock. It's simple, two positions, rugged and I suppose it wouldn't be too much to have to fuck with. But I've never needed one. Did own a couple in the past, a set screw block and a Syrac I think. Got rid of both, the set screw model was the absolute worst. Besides, most of my gas blocks are under a tube and can't easily be reached. Now if this is a RANGE TOY, or for competition only, ignore all I say. Play with it all you want, get it to be "just right" for each load if you like. If this is a DEFENSE WEAPON, stay away or call Noveske and talk to them about the use and limitations of the switchblock.
Anyway, OP is interested in mitigating gas face. A gasbuster is cheaper and simpler to install and I guarantee he'll like it better than the stock one or whatever he has on it currently. They'd be nice even if they didn't affect gas. You can try the RTV, it's worth a shot and won't hurt anything. And make sure you have the proper buffer in it, most use standard or H1 but look up a buffer weight chart online. If need be, install a new spring if the one in it is weak. I'd get a couple Wolff+ buffer springs just to have on hand also.
There are gas piston models that do away with gas in the action altogether, like the HK (I'd actually like a couple of these myself, only piston AR I'm interested in) and one other option that I hear of is a modified upper (and bcg?) that vents gas out the side of the upper. Don't know who makes that or how well it works, but not keen on having holes in the side of my upper or gimmicks on a weapon. But for a range toy it may be okay.
Good luck and remember to not overthink this. 15 AR's I have, and each have gasbusters, standard gas blocks, and each one has the appropriate buffer in it. Zero problems. I do get a bit of gas, yeah, but not enough to bother me, not enough to sting the nose or eyes. And I didn't sacrifice or put reliability of my rifle in jeopardy to achieve this.