What about EVGA Grphics Cards?
SysDef 4,1 is possible for without deleting AppleTyMCE.kext - it is what I use on my X58A-UD7 build. 5,1 definitely checks for ECC RAM on boot and will KP if you use non-ECC RAM. The AppleGraphicsPowermanagement.kext I do not know about. Might only be needed because of the dual 280X cards. I would try with it in place and use the 4,1 SysDef to begin with and see what happens.
For editing your existing SysDef, mount EFI partition, backup SMBIOS.plist somewhere safe.
Download and launch Chameleon wizard and edit the SMBIOS.plist for a MacPro 4,1 system definition.
This is something to ask of one who has dual cards. I only need one GPU to run 3 or 4 screens and have never had dual cards installed.@GoingBald
I seen that you recommended in another thread to try adding <key>ATI Ports</key>, followed by <string>4</string> (which is the # of ports my card has) to the config.plist.
My first question is: if I have two cards, would I replace '4' with the '6' - which would reflect the total # of ports (I doubt it, but I figured I'd ask).
Oh yeah, I tried Mac Pro 4,1 and 5,1.
Hijacking the thread: Do any of you know if the R9 290X (4 GB) is working with 10.11.3+ (OOTB)?
Thanks your your advice! Have one of these cards and am wondering if I can use it for a special build.