Never really expected it but since Bronya published a new Yosemite AMD kernel...
Works quite well on the Precision 670! axmanA's contribution to IOPCIFamily kext was again necessary to fully support my GeForce 9800GT with MacPro3,1 SMBIOS profile. For the rest, it's just the same kext pack + DSDT.
Yosemite was installed the "usual" way now: manual creation of USB installer + Clover or Chameleon bootloader on USB installer. I used Chameleon r2401 with kexts copied to S/L/E, copied the extracted kernel file to /S/L/K and rebuilt the cache. Yosemite can then be safely booted (to avoid white/gray screen).
With Chameleon bootloader, use attached kernel (placed in /S/L/K or at partition's root as mach_kernel), attached /Extra and replace /S/L/E/IOPCIFamily by axmanA's modified kext. Then rebuild the cache with:
sudo chmod -R 755 /S/L/E
sudo chown -R 0:0 /S/L/E
sudo touch /S/L/E
sudo kextcache -Boot -U /
Bronya_10.10_kernel_RC2.1.zip axmanA_Yos_IOPCIFamily.kext.zip Precision670_Yos_Pack.zip
With Clover bootloader, same idea with kernel/kexts/DSDT (to be placed in all appropriate /EFI/CLOVER subfolders) and use the attached config.plist file:
config.plist.zip
Many defects remain, mostly graphics related. However, it's early days for Yosemite AMD kernels so things will improve in coming weeks and months. It's a start and a rather good one I think.