You need an adjustable gas block. Subsonic rounds have less energy and that means less gas to work the gas driven action.
Adjustable gas block run the gamut from inexpensive to expensive as well as infinite gas adjustment to just a few settings. Do your research and get one that meets your needs.
Adjusting the buffers/bufferspring is not what I would recommend. The recoil system on the AR is meant to be pretty much dirt simple. A properly set up AR can mix and match buffer components (ie Rifle, Carbine, mid-length) and still work just fine. It's meant to be that way so the Army did not have to throw out a whole bunch of parts when they went from Rifles to Carbines as well as the various gas system lengths.
Adjust your gas to get a reliable lock back on an empty magazine. Once your do that you can fine tune it by looking at your ejection pattern. Overgassed works reliably but wears on the weapon more while undergassed is not reliable but is less wear on the system. All your doing in fine tuning your ejection pattern is finding that balance between reliability and wear. Most AR15 owners will never be at the point the wear makes a big difference.
Use the buffers to fine tune the ejection if you please but remember, you are tuning the rifle for one specific lot of ammunition at the current condition of the weapon. Change the ammo and you change the tuning point. Clean weapon vs Dirty Weapon will also change the tuning point. Even the next lot of the same ammo could be different and no longer give you that exact, perfect, ejection pattern.
