I have the FI 18" + Berk HFCs + Motordyne M370 combo, now thanks to B1ades
, I am keeping my eyes opened for the Stillen G3. Per B1ades, you wouldn't hear the G3 over the exhaust from inside but you do hear the air being sucked in from outside the car.
You mentioned something about loudness, you can always start with the Nissan exhaust, then get the berks, then G3 and go from there. This is close to what you should expect with the FI exhaust, Berks + Stillen G3