Yes, they do apparently overclock better, which precisely illustrates my point of them having the copper cores. Copper cores overclock better, and don't heat up as much.
As for all 1.2Ghz and up being copper, maybe I had an older one, but I had an 1.2 OEM show up in a crushed box, courrier's fault, and the chip was completely cracked in it. I figured I may as well crack it open a little more, who would know... the core was definately aluminum!
Besides that, if there was no difference in OEM and Retail chips beyond warranty, packaging and the HSF, why would only a few batches like AXIA and AVIA overclock well?
One other difference, when I got my replacement I went for a retail box, paying the difference... now I'm not about to break it open to verify what the core is, but I can tell you it came with the L1 bridges uncut, which is a definate bonus. I know other people who bought retail boxes and their L1 bridges were uncut as well. I also know that they can overclock their chips (I personally don't overclock) and are getting next to no increase in temperature on the CPU.
Even if the newer 1.2 Ghz and above chips all have copper cores, the retail ones are better, no doubt about it. You can get lucky of course and get a good OEM chip, especially if you look for the AXIA and AVIA batches, but it is largely luck of the draw.
Wayne