It may cost you few hours to study, few minutes to dump the ROM, about a min to run the script and create the Mac EFI ROM that 100% fit your card, and few more minutes to flash the card.
However, if you want to quiet card, reference 7970 obviously is not an good option. Anyway, no matter which 7970 you have, you know is that quiet enough to meet your expectation. It's on your hand now. You can now just install that into your cMP (it's 100% OOTB) and do your normal job to check if that's too loud for you.
For 7970, it's not that hard to reduce the noise, but it will cost few hours of work. Basically what you have to do is go to windows, and then perform lots of stress tests to check what's the minimum voltage that the card can run stalely, also find out what's the min fan speed that can keep the card away from overheat. And then edit the ROM image to create your own "quiet version" ROM.