I'd prefer to have them coat it inside and out, unless there would be a clearance problem with getting the baffle back in and/or attaching the megaphone onto the 2-1 headpipe collector. I'd have to call them and ask how thick the coating is. I could do a little grinding if necessary.
I know that WB is now offering the E-pipe in black. Any idea if HPC is doing the coating for them?