Did it work OOB? I just got a Nvidia GeForce GTX 1050 that I can't get past the "gIOScreenLockState" screen so I'm going to take it back.
If the RX 550 works for you, I may go pick up a similar AMD card.
Update for RX 550 in High Sierra.
Fake ID in Clover with and without inject ATI.
I tried several fake ID settings in Clover Graphics Settings. 0x67EF1002, 0x67FF1002, 0x67EO1002, 0x67CO1002, 0x67DF1002, etc.. So far nothing has worked to get any support for the card, I also tried with some of the Vega IDs in the 1000Controllerkext but no luck there as well.
Manual editing of kexts. (Adding the Device ID to the info.plist of kexts)
I used High Sierra because it has more drivers for AMD graphics cards than Sierra, however there is no RX 550 Device ID anywhere in the graphics kexts. I edited every set of kexts that are currently used by the RX 460-560 or Baffin cards ie... AMD9500Controller, 9510, 9515, 9520, and even the 10,000kext for the Vegas. I also, edited the 4000kext, the 4100, 4150, 4200, 4250, and 5000kexts as well. The closest I got to any support was loading the AMDRadeon4000HWServiceskext but the 4100kext and 9510Controller didn't load at all so there is never anything more than 7mb cram support and it always runs on default display drivers.
Injecting VROM using clover.
I returned the card but then remembered that I didn't try the vga.rom injection method that sometimes works for unsupported cards. I extracted the vbios for the card and renamed it 1002_699f.rom and placed it in the Clover/Rom folder. I selected Load Video Bios in Clover Graphics settings and tried with inject ATi and without. Unfortunately, the card is not picking up any support at this time. This is with the latest Dev. Beta version 6 of High Sierra I also tested with the HS 13.1 public release as well. So far no luck, not even partial support.
Maybe Apple will include these cards for support but for the time being there is nothing in the drivers that indicates anything for now.