Having the "shaders" value at 0 means that it will default to whatever the native value would be for the GPU so changing that won't do anything.

OP; What is the exact model number on the sticker on the back of your GPU? It should look something like FX-797A-**** If it has a V1.0, 2.0 or 5.0 list that too.

I would say you might need to flash a new BIOS to your card but getting ~600 is higher than most people(including myself) get without a BIOS flash. This tells me that you have a setting issue. If you tell me the model number I can help tweak it.