Is it legal to add carbon fiber panels to your car? So instead of more power, make it more lighter. Carbon fiber hood, fenders, doors, front and rear bumpers, seat buckets, roof (cut metal one out), floor panels (cut them out too), spare wheel well (that one as well). Add some bracings. I think that should amount to $6000.
I had an R33 GTS, and the mods I did were Cusco front strut tower bar, Apexi Power Intake (clamped a PVC cone shield to it), crank vented filters, Summit Racing red ignition cables, better ground (four 8 gauge amplifier cables from ground to the engine) deleted the resonance box on the intake pipe, and made a cold air duct to the Apexi Power Intake by cutting out a hole where one of the turbo pipings would be in a turbo R33. It sounded incredible, the exhaust was stock but the intake noise was awesome. It wasn't fast, but I was really happy with it. All were done under $300 so imagine what $6000 will do.