Not really viable, not only because of byproducts, but because of quantity.
so you saying that there is a byproduct that only way to acquire it is one of the mentioned gregtech ores? (iron, copper, tin, gold, lead)
cause when I was looking at it I didn't see any item that couldn't be acuired after changing to railcraft poorores. I'm asking cause there is a chance that I missed something.
(I mean item that can be ONLY acquired by gregtech ores. I don't mean item that one of the ways to acquire it are gregtech ores.)
I just don't want to be in situation that I find that I can't craft something just becouse I replaced gregtech ores with poor railcraft ones.
and one of the reasons I want to do it is I want to have it like it is in railcraft, many ores but not much output, grindy way.