There's still 720 to go around though
glitch in Sapphire HD 5770 Vapor-X cards seems to be responsible
disabling 80 shaders, dropping their number to a total 720, which is
what you get on an HD 5750.
Although the shader count is slightly lower, the clocks were not
affected, or any other part of the card for that matter. According to
Techpowerup, affected cards use BIOS version 012.014.000.004 and it is
possible to use a different BIOS version (012.013.000.001.034705) and
unlock the full 800 shaders.
You can do it yourself, or you can get in touch with Sapphire support
and it's probably a good idea to do so, as you shouldn't fiddle with
the card's BIOS unless you know what you are doing.