Regulating the gas system can be very worthwhile. Rifle function, of course, is the priority, but you'll probably find that there's plenty of that to spare if you go with longer barrels. A 24-inch barrel with the same gas-port size and location as a standard 20 inch will convey additional pressure to the operating system. The reason is that there's more gas contained in the bore for a longer time. Excessive pressure means excessive functioning--the bolt carrier moves too soon and too fast. The first affects spent-case condition and also can restrict attainable bullet velocity before a structural primer failure, and the overall speed the carrier travels can batter associated parts. Broken bolt stops and hammered cams are symptoms of overfunction. If we can delay bolt unlocking without compromising cycling reliability, that's bliss. Most AR-15 match rifles get their gas port moved forward, toward the muzzle, an inch or so when they get a 24-inch-plus barrel. They'll often also get an adjustable gas block to regulate flow to the gas tube.
Beyond a good barrel, the majority of parts options for AR-15s are there to configure the rifle to the task and to the shooter's preferences. All the other trickery on competition AR-15s is there only to make you shoot the rifle better. The whole trick to target shooting is hitting the target. Make good choices on your personal specifications, and the AR-15 gets really easy to shoot.
I expect my competition AR-15s to group under five inches at 600 yards, around two at 300 and under one at 200. Those are 10-shot groups fired prone with optical assist.
In the final analysis, if you want an AR-15 that shoots really well, here's what you do Free-float a good barrel, and feed it good bullets. That's it.
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