Besides headers theres dinan bored out throttle bodies and schrick cams. Those mods probably won't net too much hp but could make the difference if you're looking for a specific number while staying NA. Vishnu meth is also an option.
Melbourne Red E92 M3 DCT Sparco CF/Alcantara DTM Seats.KW3 Coilovers.Apex Wheels.OEM Performance Spoiler with CRT Theme & Splitters.Turner Subframe Mounts.Vorshlag Camber Plates.RD Sway Bars.AD08s 265/295.15 lb. Braille Battery.Catless Gintani Race.ESS Tune.Dinan Pulley.MS2 Intake.100 Octane Mix.Schroth Harnesses.Challenge Harness Bar in MR.Challenge Toe Link.Krics R40.Turner Studs/Pedals.SS Lines.Rear Seat Delete & more I can't remember