It would help if we knew which BRAND of R9 280 which can be flashed with thr AMD HD 7950 ROM.
Do you have a Sapphire, Gigabyte, MSI etc. R9 280 ?
( XFX is not recommended as their cards often vary from the reference design )
if you want quick, helpful replies then go to your Macrumors member PROFILE and select " SIGNATURE ".
Type in all the important information about your cMP set up - my SIGNATURE is at the bottom of this post as an example.
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Your R9 280
CAN definitely be EFI flashed.
It is not that difficult if you first read ALL the posts of this link.
7xxx rom creating test.
forum.netkas.org
I also felt threatened by what I thought was a very steep learning curve regarding flashing AMD cards for Mac EFI but have since successfully flashed 6 different AMD GPUS.
If you want to try, PM me , I can take you through the process.