From my own testing the optimized shaders actualy come from the drivers. When the drivers install they look at the device ID and from there tell the OS which path the drivers are to install ( and which optimizations each core uses) Now i did 3 tests with an XT bios , a pro bios, and an XT bios W/a pro's device ID. And if ya look at them ya see that the only one scoring higher is the one that appears as an XT. and even tho the card had an XT bios with a Pro's ID it scored the same as a Pro.
http://service.futuremark.com/compare?2k3=3834595 XT bios
http://service.futuremark.com/compare?2k3=3819096 XT bios w/pro ID
http://service.futuremark.com/compare?2k3=3821804 Pro bios

I must note that all i did was take a normal XT bios, and edit the amount of card memory with rabbit to 128Meg for the XT bios. and thats why the card lists as 128, instead of 256