OK, most know my constant PREACHING on using the smallest size pipe for the needed flow. That said my statements are regarding naturally aspirated engines. Turbos are DIFFERENT animal. They what the highest pressure differential possible.
Now, David, you are designing the system to use the stock exhaust including the catalysts. As discussed before twin 2.5" will support your power. I would not worry about an X, Y or H pipe. You could run a single center 3.5" or even 4" pipe. That would split back into twin 2.5" pipes and mufflers.
Now, remember the stock cat outlet ID is small. You may want to build replacement cats utilizing the available metal subtrate cats.
