You'll just have to make the changes to your active config.plist, which should usually be located on the EFI partition of your boot drive. According to the
Radeon Guide, you'll have to configure your GPU as follows:
- FakeID / ATI = 0x67E01002 or 0x67FF1002 or 0x67EF1002
- Inject / Ati = true
- FBName = Dayman or Guariba or Radeon
- FixDisplay = true
For the FakeID value, all 3 possibilities
should have the same effect. If one doesn't work properly, I'd try the remaining ones though.
For FBName, all 3 possibilities should be working fine. The latter will make your graphics card run without injection (that's what you already have) while the first two will hopefully make it run with a 100% matching framebuffer. This also allows you to enter a proper display name (e.g. "Radeon RX 480").
Btw, what I'm most interested in (well, besides acceleration obviously

): Does sleep work?