From what I've seen, P5 Firefly rendering engine produces much better and more realistic skin tones no matter what model is used. Not long ago someone contrasted Daz's Victoria P5 render with the P4 rendering engine. The consensus of those leaving comments on the thread was that the firefly engine produced much better results. I don't think your problem (unless your software is unpatched) is related to whether you're using a Daz model or not. Have you checked your lighting? The way the 2 engines render lighting may have more to do with why your finished results. If you post an example here people may have a better idea what's going on.