RE: Hydro Marine X-Ray
I used some ali pipe it was same diam as the header pipe so is bigger than the stinger,you can go a tad smaller but you need the extension to be bigger than the end of your pipe or yes it can affect your pipe pressure..Maybe try 2 sizes up from your stinger diam and use a silicone exhaust coupler to join the two as i did it will last a while because it doesnt get as hot that end especially if your running water through the pipe as you say.
Mart