according to an response in this thread.
https://discussions.apple.com/thread/5880374?start=0&tstart=0
the mac pro require apple firmware on the cards. from the link:
"• Apple-firmware 5770, about US$250** works near full speed in every model Mac Pro, Drivers in 10.6.5. (This card has just been discontinued and is no longer available from Apple dealers. There may be some new, old stock still available at resellers)
• Apple-firmware 5870, about US$450"
from the thread responses the cards firmware will enable the "Boot up screens, Safe Mode, Installer, Recovery, debug screens, and Alt/Option boot screens" and may have different power wiring than a PC card.
Based on the thread there may be special setup ie flashing the card, cabling issues.