If it's a cat-back system it won't affect the stock ecu tune. It will have sensors before and after the cat converters so it should just flow more at high revs and sound awesomeness at idle. If the original Nissan system had reactive mufflers at the rear and not straight through you might loose a bit of bottom end torque but gain more power at higher r.p.m. A proven hks product will just bolt on and be mint for what you want (moar top end and moar noisy) unless you've changed the tube size.