Bigpinkdragon pretty much nailed it.
With Bios flashing on a GPU, you are either doing it to fix compatibility issues or changing some parameter of the cards performance. Such as default clock speeds, voltages, unlocking shaders/cores, etc..
Not all cards have a flashable performance Bios, some cards were able to be unlocked such as the AMD 6950 flashing to a 6970 or 290 into a 290X. However, these results are not typical and not all of these cards will do it.
Also, unless you have a card with a dual Bios switch, I wouldn't recommend trying to flash the card unless you feel 100% confident.