I am working on this at the moment...I had a Windows only 4870 and flashed it to add EFI portion so it would work in the Mac.
In answer to the question from page one of how to extract your original BIOS in Windows Boot Camp use GPU-z that extracts the BIOS..
extract it ..name it ORIGINAL or similar, copy it, work wit the copy and and keep ORIGINAL safe..so if you screw up you can go back to original settings.
On other mac rumour forums there is a post with a working ATI 4870 firmware with EFI section for MAC Thread title was something about dumping the 4870 ROM.. or Golden thread or similar but I won't post a link to it in case I am breaching rules here