you could get to about 180hp from the factory 165 at the fly without spending big dollars like cold-intake, good NA extractors & exhaust, and a play with the timing, once big dollars have been spent on cams and a new ECU you will only be looking at 200 maybe more then that and it could be a bit of a pig to daily drive. But as mentioned above, when the spec r makes 240hp at the fly in factory form you are really wasting your time and money, even an engine transplant still works out more then buying a spec r in the first place.
You could +t but then you will only be able to make basic spec r power before blowing the DE up.
sorry to sound all negative but i learned that one. i used to have a spec slow and in then end just got a spec r
If you are dead set on NA tuning, the best starting point is to get a VE head and bolt it on and make the bottom end larger think about making your engine SR22VE, using that as a starting point you will be able to chase larger power figures.