Y pipe or custom H?

[AAAlovesME]

Im curious what would make a difference in the power curve of the stock Y pipe VS a custom H? Or even 2 straight pipes from the headers back w/catalytic converters. If im doing the H route or even straight pipes, do i need to have 2 catalytic converters per side per pipe?, Or just 1 on each side? Im trying to gain the most but stay legal. Im not going to run exhaust & im routing out both sides, already made & mounted these bezels on both sides...