2018 hatchback sport owner:
-I’ve always run premium so i unfortunately can’t tell you how much of a difference it makes.
-I (used to) have a Hondata tuner to add +6 PSI of boost, nothing crazy but definitely noticeable and a lot of fun. Probably the easiest/most cost effective method of getting real performance gains.
-I have a Greddy catback exhaust, sounds decent, better value than the other options imo (borla, magnaflow, etc). My favorite thing about it is the bigger exhaust tips though, fills in the exhaust cutout better and looks much nicer.
-a Hondata or Ktuner tune is the one i’d definitely recommend for performance, and a new exhaust is definitely a nice change. I have an Injen short ram intake, the extra induction noise is nice but i probably wouldn’t have bought it new (i got it used for $100).