Yeah, I went through an experience where I flashed an x800gto to an x800xl specs. It worked for almost a year, then died. I don't know exactly what happened, but now I know that you should not flash the bios on a graphics card since you more than likely will fry the card. I know this might be sad news to you, but I would not even attempt it.
The GTX 260 has less vRAM (896MB vs 1024MB), less stream processors (192 vs 240), smaller memory bus (448-bit vs 512-bit), less TMUs (64 vs 80), and less ROPs (28 vs 32) compared to the GTX 280. I think it's pretty obvious that the chance of success here is less than 1% (I never say never though).
Oh yeah I forgot, the GTX 280 probably uses faster rated memory with different timings compared to the GTX 260... after reading all the inherent hardware differences you still plan on going through with this, then I wish you the best of luck.
Didn't you just read all the above? There is WAY more stuff disabled than just 48 shaders, even if you could enable the disabled shaders with a BIOS flash, you would still have to enable the disabled ROPs, TMUs, pray to god that vRAM timings are somewhat acceptable, and hope that you don't run into bizarre issues for having less memory and a smaller memory bus.
I'd say you got a one in a million chance of this actually working in one card (you would have to have something like a pre-production sample that was disabled through BIOS and not laser cut), let alone working on both...