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Is there even a difference in forgings? I've heard metallurgical differences making a Colt superior... but any evidence or truth to that?
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The difference is that Colt placed very high quality control requirements on its forgings, which were intended from the beginning to be military issue firearms, and that in turn made them more expensive for Colt to buy.
The makers of clone AR15s (a) did not intend for their products to be made full-auto, and (b) they were attempting to make a clone that they could sell for less than a factory Colt. Thus they had lower QC requirements, and could buy them for less $$$.
While today there are brands of AR15s that are superior to Colt, before May 19, 1986, Colt made the highest quality AR15s (and M16s). All of the clones sold for less $$$ because they cost less $$$ to build.
(The one exception might be the stainless steel Group Industries receivers, and that is only because of the material from which they were made. Virgin GI receivers were notoriously out-of-spec.)
Non-Colt AR receivers have been known to have the occasional void in the forging, which is a problem if they need to be repaired. That issue certainly is rare -- I've only seen one myself in 30 years -- but I've never, ever heard of an issue with a Colt forging.
YMMV.