"The Airbox" Maybe an exhaust system with 200 cell cats? Bilstein coilovers (I have them) and mods for faster shifting. That's really all I would do for performance upgrades.
2001 M3 Coupe TiAg/Black
PSS10 Coilovers | TMS Camber Plates | BMW Performance Shift Knob | LED Tail Lights | AP Headers | Two New Rear Bumpers | 219M Wheels |