I haven't heard the 5Zigen in person but i've heard the Tanabe and the Tanbe ounds like complete and utter ass. That shit is so fucking loud it's ridiculous. Maybe it was that particulae Tanabe I heard but all he Tanabe's i've heard, on any car, they've been loud.
IMO, if you want decents gains with minimal noise, go with an RS*R Ex Mag.