Yeah, I'm a lefty and shoot a suppressed AR (a couple actually). Even low velocity rounds like 300 blackout subsonic will throw debris from the ejection port in your face. I've spent a lot of money trying to "fix" the problem.
I've tried gas blocker charging handles, adjustable gas blocks, a forward assist replacement with a special port in it, a piston upper, and the list goes on. I have a left-hand upper and it does solve most of the problem. The adjustable gas block also helps a little bit and can also keep the BCG from slamming into the buffer, too (nice side effect).
Mostly, I think the most sound advice I have is: deal with it. If it's a big distraction while you're bench shooting, a brass catcher actually eliminates A LOT of the blow back, but isn't real great for run-and-gun stuff.
Good luck and I hope you know that NFA stuff is a sickness -- plus, once you have it you can't get rid of it.