Your next mod should be the M3 chassis parts. They really tighten everything up and improve the handling ALOT. Don't forget the front strut brace.
Camber plates with coilovers will also be a big improvement.
Finally, if you really enjoy tracking the car you should sooner or later look into a BBK. Gives you SO much more peace of mind on the track and cuts some tenths of your times away too of course. You can oftenly find good deals on BBKs in the used parts sale section if the price steers you away.
Performance Seats, Exhaust, Splitters, Pedals, Steering Wheel / RB Turbos / M3 CF Roof / Brembo GT BBK 355/345 / Forge FMIC / QUAIFE LSD / Bilstein PSS10 / M3 Suspension Parts / Powerflex Bushings / Vorshlag Camberplates / RE Toe Links / LeatherZ Gauges / Extended M3 DCT Paddles / ER Sports OC / AR OC / Aux Radiator / AR DPs / Alpina TCU Flash / COBB Pro-Tune