Bough brand new probably adds the most sound to your frs/brz to be honest. Header sounds great but cat and resonator are what are probably the most restrictive. I noticed the biggest chance personally with front/over pipe. Very aggressive sound to it!

Will include new header-over piper flange nuts and bolts.

Resonator / Cat delete (disclaimer: be aware of state emission laws). Definite increase in gains/airflow!

C-clamp connection makes for an easy install (30min tops depending on tools). Would suggest taking header off first to get over pipe in and out first.

Stainless Steel. One of the best brands out there. Part was barely used no more than 2k miles on it. Couple months old.