Ah hell, now we're talking, cause I won't own DI anymore with how good piston AR's have gotten.
Rainier sells Adams which I having been using gas piston AR's for about a decade now and feel are one of the best systems out there in my opinion and they make high quality melonite treated uppers for reasonable prices and use myself.
For what you're looking for a med to heavy barrel upper with room for optics and folding BUIS they've got options for you. Rainier has the 16 inch stainless, fluted barreled, competition upper on sale right now which is designed around precision shooting and perfect for optics.
You're still going to get a very accurate upper with one of the non-fluted government profile barrels though and longer lasting since it's melonite treated. Per say you wanted more of a fighting upper.
I personally like the EVO models and plan on using one for another AR with there thinned down forearm without the side and full length lower rails on the it that have never been useful to me and make the grip less comfortable. I've only ever needed to mount BUIS on the top, sling swivels on the side front rails and room for a bipod down on the bottom front should I want it. The EVO are designed around that.
Here's Adams mother site,
and here is another dealer RifleGear that my neighbor dealt with and was good and has good deals,
AR Complete Upper Assemblies