I'm new to ARs but not to rifles generally. I don't think the FF tube is necessary. On rifles with wood furniture maybe—but the AR? I don't think so.
I don't think the Delta Ring is a good design. I do think that the relative looseness of the two piece M4 style guards (or any guards that move) are a problem. A tight fitting two-piece quad-rail takes any of the "play" out, and I think may actually be a benefit.
Even a FF tube flexes about and actually applies effective leverage—in the sense that it is capable of producing amplified force—to the overall geometry of the rifle in an inconsistent way. There is enough variable movement in the rifleman that is only compounded by anything on the rifle that moves or flexes.
Matching ammo to the barrel is huge I think, The orientation of a Cup/Core projectile has a huge effect, and a bullet being driven backwards (FMJ) has a more irregular base resulting in a less stabilized spin than one being driven "open-tip forwards" (HP). With a good crown on the barrel a flat base bullet will be best. I believe that a boat tail on an FMJ is a contradiction in terms and only serves to lengthen the projectile requiring a tighter twist... it is not a make good on a bad crown. However it will raise BC.
The sighting system it's self and perhaps more importantly the way we attach it, is probably the biggest robber of accuracy. If you're looking for SUBMOA all the time, then a scope is obviously called for. The ability to mount the rifle consistently without canting it (even at the bench) is also imperative with sights that are this far up from the bore.