With just a little roughing of the FRP surface, your problem with silicone and the fiberglass will be removing it, if (God forbid) you ever had to do so.

The advantage of the smoother surface (except possibly where the silicone is adhering) is that if you use a clean-out, you can wipe any scum off.

PVC also might not like your shock treating it with high concentrations of chlorine (but I'm not very good at inorganic chemistry) ;^))).