I have an S12. Thus far it has ran great on everything I’ve fed it, EXCEPT the cheap Winchester birdshot/skeet loads you can buy in bulk. The cheap Federal shells will get out of that 20rnd drum as fast as you can pull the trigger without so much as a hiccup.
All ‘high brass’, high powered shells have done great. Never have messed with the gas plug any, because I haven’t had to.
Definitely checkout the Saiga 12 forums for lots of good info if you haven’t already.
I’ve seen videos about adjusting the gas piston 1/4 turn at a time and then test firing and keep doing that until it runs on whatever you’re shooting. not much good if you depend on it working immediately after cleaning it , IMHO.