BESIDES flash tuning for a lower output of the motor, N/A engines can be limited in power by intake restrictors, something used a lot in the motorsports world.
I voted turbo, because your gonna love the car N/A, get smoked by some E46 M3 or Evo and your going to want more power. Install the turbo, run low boost for now then when you can build up the motor, turn that shit up