Yes mate very different from the Nissan and Subi comminuty. Some parts are interchangeable with Evo's ie brembo brakes if you use Face Lift front uprights. Rotors are another common.
Yes a good thing to do is change the fluids, all of them, oil brakes AYC and Diff. Consider gearbox though that may not be necessary.
A simple and most cost effective change is replace the O2 sensor. They really don't last long (60-80thousand) plus it'll have a dramatic effect on your fuel economy, ie improve it.
Yes exhaust intercooler and re flash will see good gains. See SteveTME he's the resident tuner in Sydney