I am fed up with the nvidia driver issues and am looking to replace my GTX-950 with an ATI Radeon based card that can be used almost out of the box with Sierra and hopefully High Sierra. I'll be using the graphics cards primarily for photography-based apps (Lightroom and photoshop) as well as very light video editing, so support for two monitors are important. I am not into gaming. Any suggestions?
Your best options are R9 270X and R9 280X cards if you can find them. These two cards work OOB (Except for XFX cards it seems they are not so easy to use on hacks) without the need of any kext modification.
For having two displays working you might need to create a custom framebuffer though.
There is a new way to avoid the black screen issue by using Lilu.kext v1.1.6 and WhateverGreen.kext v1.0.3 which work pretty good with almost all versions of AMD GPU's including the RX Series and the support for of 2 displays without the need to do anything special. I tested those kexts on my R9 270X and it works fine with two displays without any ATI/AMD injection or framebuffer modification.
There is also a new generation of AMD GPU's called Vega (as you might heard of) and they seem to work OOB without any issues. (That's what I've heard I haven't tested them yet myself).
For more info you can look: WhateverGreen Support, AMD Polaris and Modification of AMD FrameBuffer.