SAPPHIRE'S BIOS for the HD 5770 Vapor-X cards seems contain a small glitch which is responsible for disabling 80 shaders.  The BIOS fault drops the total number to to 720, which is the amount you get on an HD 5750.

The clocks nor any other part of the card were affected. According to Techpowerup, affected cards use BIOS version is 012.014.000.004.  Other versions of the BIOS  (012.013.000.001.034705) are not affected and unlock all 800 shaders.

Although users can fix the problem themselves with a BIOS flash, we still suggest that you can get in touch with Sapphire support.

You can read the Techpowerup article here .