I have always run the front only exit on my 840 X2 with a stock waterbox. It isn't loud stock. I have another hull the dual scupper incorporated in it already like you mentioned. It should work great I hope. It does leave poop alover the side of the hull. It wipes off easy enough with WD40.
You might want to try a 650 style Kawi waterbox it you are going to do it. They are relatively cheap and you can run it in the top off the chamber with a standard 90 hose then run it straight out the side below with no joints to get it out straight. This is a stock one with the rubbers on it but you get the idea.