Your SR20 does not 'need' any back pressure at all. If it is in fact a turbo engine, then zero back pressure would be a good figure to shoot for.

In NSW, you must have the exhaust exiting behind the rearmost openable window, so exiting on the drivers side in front of the rear wheel will probably be just fine, as long as it meets the noise regulations. I think that they frown on side exit exhausts that eject gasses onto pedestrians when the vehicle is parked, so i think that the passenger side is a no no.