AMD METAlL Supported cards that can be EFI bootscreen flashed
HD 7950 R9 280
HD 7970 R9 280X
RX 580 8gb, - can only be flashed by " Mac Video Cards ".
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Cannot be EFI bootscreen flashed
RX 480
RX 560
V II
Vega 56
vega 64
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There are more .. Google ( is your friend ) "Mac Pro Metal Supported GPUs.
( NViIDIa cards can only be used with NVIDia drivers in High Sierra. )
Also, please make a SIGNATURE to describe your cMP setup - this saves us having to ask million times about your Mac Pro.
AS an example my SIGNATURE is at the bottom of this post.