Without macrosurface (normals) and microsurface (gloss/shine) details, models are, of course, going to look bad.
I've addressed the issue in the past on several occasions, I should probably just make a copy paste for it at this point. Anyways, for many models that were never designed to have -shine maps (covers all of retail and the UEF fighters), PBR isn't going to look good without people going back and making the textures (Hell, the current UEF fighters still don't have normal maps!). In this case, your ideal solution is not only nigh impossible, it's highly impractical given the nature of the issue.
I ask you to take the time to read my previous posts in this thread and others related to PBR. My knowledge on the matter is, of course, not perfect and in fact evolves based on my observations of other rendering engines, research, and things I encounter in real life (****ing vantablack...). I'm doing my best to develop workflows and guides to fixing assets up for PBR in FSO.