from what my research indicated, all you need to do is switch off the SRGB flag for the Roughness and Metalic in UE4’s texture editor, even though it probably won’t still look exactly 1:1 between the two apps. Once UE4 compresses the image you will automatically loose quality, and the only other option I can think of is to turn mip-mapping off on your textures, which isn’t really an option for games. and of course you need to set your compressor on your normal maps to Normal Map Compression.
I have redone all the texture work I had done up to this point in SP, and over all I am quite happy with it, it takes a fraction of the amount of time to paint these type of maps in SP, than it would the traditional way, though if you want to create specific image based detail, I find that Photoshop is still the best way, unless you just want to project an image on to your mesh in your standard 3D app and then bake that to the UV, which is also an option.