Thanks, I’ll probably go with b) indeed.
I’m not very proficient in the material stuff yet, but I try to learn more. I’ve read about texture lookups, shader instructions, etc. and it seems like a function like that can be quite expensive on 4k RT, since it gets the value 5 times from it and does these calculations.
I also found another thread with related topic here:
https://forums.unrealengine.com/development-discussion/rendering/1502226-material-cost-texture-lookups-vs-shader-instructions