pc cards come in many variations, and differ in ports to the Mac versions.
Therefore, it it is possible that after flashing the card to Mac.efi not every port will give signal.
You have the biggest chance of success, with a reference model of a videocard.
The link to the page could be helpful, the efi part that will be added to the vbios part.
The efipart for every 5850 is therefore the same, that's why it is so important, that you post the original rom of your videocard.
This way you get a completely custom rom, for your videocard.