+1 on rear loop, particularly with shifters where the cable isn't routed under the bar tape. I often used to just replace the rd housing loop. It's the tight radius and length of the bend that makes it wear faster and contribute more to poor shifting. Cables that exit front of the shifter have only a single large radius 90 degree bend to the frame stops so there's little drag or wear compared to cables that make two tight bends under the tape, so now I find this as critical as replacing the rear loop.
I have not found any shifting difference between metal, plastic, or noodled ferules. I'll use metal if I have them but plastic if not. After cutting housings, I square the ends with a grinding wheel or Dremel and trim the outer casing to ensure the housing wires bottom uniformly in the ferules. Pro racer mechs don't do this so I doubt it helps significantly, but I do it anyway.