This strategy would be unthinkable with Windows. It would take a lot of wheeling and dealing to get Microsoft to go along with something like that, especially if there were a chance that this change would be unstable or difficult to maintain. Microsoft would surely request large sums of money from the chip companies just to consider this approach. Whether or not that would prove to be a risky investment, it would require Intel and AMD to shell out more than just programmer power. They'd have to shell out bucks in the hope that someone would purchase their chipsets specifically to get the extra performance of the special Windows version.
Quality assurance is inherent within the open source development process.
And the only thing AMD and Intel need to invest is effort. While it would be possible to cut deals with commercial distributions, it wouldn't be necessary. As I suggested above, these chip companies could simply post optimized distributions on a server and let people download them. Heck, this contest doesn't even have to be limited to AMD and Intel, although that's where the fireworks would likely be prettiest. Regardless, AMD and Intel, admit it.