When I bought the pdr-HX40 I bought the downpipe and clamp from Piers also.
Called RIP Rook and ordered a 4" system minus downpipe.
Everything fit together fine.
If I had it to do over I would probaly go with Rip's 5" system for visual impact. Just a thought, not a suggestion.
I would suggest that what ever you do use a straight through muffler like Allied or Donaldson, etc.
I'm sure there are other ways to go about this. Maybe someone else will say what they did.
Hope this helps,
Kent