You'll spend a fortune on NA tuning and tbh, not get an aful lot. You may as well have went FI if it's power you want.
Far better idea (provided you have ruled out FI) is to go for a set of good coilovers, good tyres, new bushes all round, and a driver training day or 2.
You'll get more from the car than a few thousand quid on NA power mods ever will.