Ragingyeti an aluminum upper would greatly increase accuracy. My hypothesis is that as the barrel heats up, the polymer threads will begin to distort and shrink or become misformed. Metal threads wouldn't be as easy to damage or heat up.

Wolverine, that is exactly what I was thinking talking about. By stiffening the whole receiver there is less flex in it. Now doubt that with optics there is a slight amount of flexing when you tighten on rings and sights and what not near the back. Thats why the ar target rifles are mostly heavy. The more metal there is to stiffen the whole thing the better right?