To add to this, here's a decent breakdown on how to get good grass in UE4 https://forums.unrealengine.com/showthread.php?3268-Nature-Environment-in-UE4&p=21370&viewfull=1#post21370
I think ue4s specular input is more of a specular occlusion/level parameter but I've not tested it out. For ue4 the simple fix is to use the metallic + coloured basecolor hack
I had this problem before, and here's an answer that worked for me. https://forums.unrealengine.com/development-discussion/rendering/28835-substance-to-ue4-issue-black-metal-in-ue4-viewport
SRGB in ue4 does not change anything. Howerver, it seems like if I enable "gamma 2.2" profile for greyscale image in photoshop, it does match the end result in ue4.
Here is my wip asset ! Nearly finished! Rendered with ue4. No sculpting everything done in photoshop and maya (and ue4 ofc) I went for something more realistic ! Render