I’m a “offline Renderer” in the industry, converting to RT … and in my experience… You def. Want to go for the Metal rough workflow, gives you waaaayyy less headaches getting the look that you want… And as mentioned before, spec works a liiitle different here like in, lets say vray… And even there… When you go with Metal roughness, you’ll have no big problems to Match your Looks with different engines, when it comes to PBR… Just remember, the “normal format” is another thing to look out for… For UE its “DirectX” format… For i.e. vray its “OpenGL” format… And dont forget to interpret your metal and roughness maps (packed or not packed) as “masks”… (No color Management correction)… Thats how you get the exact same (almost 
) look that you’ll have in lets say substance(adobe) painter and ue…
And as a little reminder… Color info-> sRgb… Everything else ("""“Data”"""") linear/or Raw … And then you’ll have a fun time with metal-rough 