Just to let you people know s939 Opterons are Athlon 64s and are not "higher quality silicon". They are clocked lower making them more stable (which is important if your running a server).
That's quite the paradox =/
It's already common knowledge that all K8 architecture is basically the same and PGA939 Athlons are built on the fabrication as PGA939 Opterons, but the opterons are generally handpicked and do use a better silicon. You've pretty much already said that Opterons are binned higher and then underclocked for stability reasons, whereas an Athlon will generally be binned at the same speed it's intended to run at 24/7. Higher speeds translate into a better core quality, and since a 2Ghz Opteron might really be a 2.4Ghz Opteron with a multiplier dropped down whereas a 2Ghz Athlon is for the most part originally only capable of 2GHz without more vcore, that really just means that an Opteron of the same speed as an Athlon has a better fabrication process