A lighter flywheel will free up some ponies for you. As long as the mods are sensible most should give better power and torque. Its only when you got with a massive exhaust bore or induction kit without a cold air feed that you start to lose some low down torque.
The aim of tuning though is to create an ideal torque curve. It varys with use of the car and driver preferences.
I would start of with the simple methods first a performance cat and exhaust to let the gases esacpe quicker . Then i would put the m50 manifold on to let more air get to the engine also using the z3 throttle body kit or a unreasonable priced big bore kit .all this is for the air to travel faster round the engine . next i would recomend changing the exhaust valves only i say only becasue you would be wasting your money on the inlet valves as make no differnce. once you have got this far i recomend that you put the m3 cam-shafts on .then i would say go and get it re-mapped once you have got this far get it dyno tested and watch the readings go up to near 250bhp and around 375pound of torque some vechicles will differ. but tuning from ther onwards get very expensive .