Ha, I just went through this yesterday after finally getting a replacement for my 6850 that I have wanted for so long (thanks miners - finally a good deal at NCIX and Never Settle to boot). I am on an older system still using AWARD BIOS with DSDT MultiBeast install so this may not work for you and I am not entirely sure what finally did the trick but hopefully you find this helpful.
Warning!!! - Anyone else trying this, ensure that you have a bootable UniBeast USB drive available - I couldn't get to the chameleon drive selection screen without it once I installed the MSI 280x.
General forum consensus seems to be Install Bootloader & Extra to EUFI partition, but I am lazy and didn't want to deal with remounting hidden partitions every time I make changes:
http://www.tonymacx86.com/401-install-bootloader-extra-efi-partition.html
Stingray454's post convinced me to take the plunge:
http://www.tonymacx86.com/user-buil...ell-uatx-4770k-ga-z87mx-d3h-dual-r9-280x.html
and the first thing I tried when I too got the boot0 gibberish was the link he referenced:
http://www.tonymacx86.com/25-boot0-error-official-guide.html
Step 1
I only tried solution 1 and it did not fix boot0 gibberish for me, but may be one of the steps that led to eventual success.
Step 2
I then tried Keintrunvu's suggestion, 4th post in that boot0 thead. Same solution referenced here:
http://quickwebgems.blogspot.ca/2013/09/hackintosh-boot0-gibberish-fix.html
I used the latest Chameleon Wizard 4.3.2 and intalled rev 2378 using the Boot0md option for my mixed disk setup. Didn't do anything else with Chameleon Wizard.
This also didn't work for me initially, but I was testing on a cloned partition that I had done some other experimenting on as well so it may be all you need.
Step 3
After this, I tried changing my system def from iMac 13,2 to MacPro 5.1 after deleting S/L/E-AppleTyMCEDriver kext (apparently related to EEC ram in the real MacPro) based on the post from Total_Nube top of page here:
http://www.tonymacx86.com/buying-ad...-our-hackintoshes-further-explanation-13.html
This worked, so then I tried applying MacPro 5,1 to my main drive (already applied step 1) and it would not work until I also installed the latest Chameleon with Chameleon Wizard. Same process as on clone drive but in Step 1, 3, 2 order instead of Step 1, 2, 3 order - hence the uncertainty if all 3 steps are necessary.
Added bonus, MacPro 5.1 seems to scale the i7-2600 much better than the iMac 13,2 that I was using (because Mac 3,1 used to mess up youtube videos and other things for my 6850 and I didn't want a Mac Mini def).
Some related info - haven't messed with my MultiBeast 6.2.1 generated org.chameleon.Boot.plist from what I was using for HD6850:
<key>EthernetBuiltIn</key>
<string>Yes</string>
<key>GraphicsEnabler</key>
<string>No</string>
<key>IGPEnabler</key>
<string>Yes</string>
<key>Kernel</key>
<string>mach_kernel</string>
<key>Kernel Flags</key>
<string></string>
<key>Legacy Logo</key>
<string>Yes</string>
<key>Instant Menu</key>
<string>Yes</string>
<key>GenerateCStates</key>
<string>Yes</string>
<key>GeneratePStates</key>
<string>Yes</string>
<key>UseKernelCache</key>
<string>Yes</string>
<key>Graphics Mode</key>
<string>"1920x1080x32"</string>
I have Award Bios set to auto detect displays with 64MB RAM allocation (some things I have read say to disable integrated graphics or leave RAM at 32MB).
I haven't edited any ATI kexts (other than for Unibeast when going from 10.6.8 to 10.8.3 and I think that was only on the unibeast drive).
Haven't had much chance to put the R9 280x through its paces, but it seems to be working fine.
One thing that may or may not have have been a result of the 280x is I now have a thin black strip between my top menu bar and the screen bezel that I don't think was there before, as if my vertical image shrank a bit - will try to figure out why/how to fix when I have the chance.
Also, my benchmark results all dropped a smidge when booting straight from SSD versus booting through Unibeast SD card through to SSD, not sure why that would be but it is very minor
Heaven 1600x900 8aa extreme 862 vs 859 - seen as AMD Radeon HD 7xxx(3072MB)
Valley 1600x900 8aa extreme 2035 vs 2020 - seen as AMD Radeon HD 7xxx(3072MB)
Luxmark OpenCL GPU 2088 vs 2090 - seen as AMD Radeon HD Tahiti XT Prototype (GPU, 32, 1020 MHz)
About this Mac sees it as AMD Radeon HD 7xxx 3072 MB, haven't looked in to having it recognized as a 7970 or D700 as some people seem to get.
I can run dual screen DVI to my Dell 2408 1920x1200 and HDMI to a 1080p TV just fine (no sound - haven't tried to fix that but there is a fix that should work). The card comes with a miniDP to DP adapter and my Dell will take DP but I can't remember where I stashed DP cable for safe keeping.
Hope some find this helpful.
Big shout out to the hackintosh community for sharing the knowledge and making this possible - as a lover of MacOS and upgradeable hardware you folks are invaluable. I am in awe of the tech knowledge some of you possess.