My rotors in my old 95 M62 were almost completely bare when I pulled the SC apart to port it and replace the SC gasket. Between the rotors being almost bare and the SC gasket blocking the outlet of the SC would probably explain a big part of why the car was probably .3-.5 seconds slower than it should have been. I know it definitely felt stronger from just porting and putting a properly placed SC gasket in.
As far as a place that re-epoxy'* them, from what I have heard on www.3800pro.com
the cost of recoating them (teflon coating -might as well if they are going to be done properly) would be about $400. Worth it? It'* up to the person holding the cash.