You failed to respond to my point. I think you didn't provide an answer to my question (How is a custom part going to significantly differ from a PC part?) because your point falls flat.
I don't know the specifics of the changes they would make, I'm not an AMD senior engineer, but they could easily change the amount of rops or other things for better efficiency on a fixed hardware platform, some features could be replaced by custom parts or various other things could be included. I also know that the 6850 is probably not the penultimate graphics technology, they continue to employ those engineers to innovate, not just to find ways to make die shrunk versions of Barts.
./snipped
Which backs up my point: If RSX, a pretty straight PC ported GPU, is relatively competitive with Xenos, why again are we expecting a console-specific GPU *chip* to offer substantially more per-area than the PC parts.
Answer? We should not.
If your argument is that RSX (launched a year later) almost competes with Xenos with help from cell. It's not supporting your case. It's a pretty clear indicator that the custom part was a significant benefit. Expecting little more than die shrunk version of years old technology on the next consoles which will probably launch in 2013 or 2014 is foolish. Even just assuming they can't improve on perf/mm with a custom part is out there.