It shouldn't be the same, but between nVidia and vendors, it often IS the same. The reality of the "production" doesn't match what nVidia intended (assuming nVidia were entirely honest)
I have 9800GT with a unique device ID and it's still 65nm.
There's both die processes available. User can only tell from the die markings and GPU-Z.
A user can go into a shop and buy a GTX+ and (using your idea) then be told they have to submit it as a GTX because it's 65nm. In theory, the implications of that mean the vendor is selling through false advertising. Maybe they are (lol) but it's far easier and less... .hmmmm.... "controversial?" to throw the categories together. Minimal maintenance.
For most of these cards, don't even need to flash the BIOS, just use the lack of clarity in the device ID and GPU model. I did flash my GTX with the suposed Cold BIOS. Some GTX+ can have the same done (same PCB)