Bernie pretty much covered it all, but I just have one little fact to add. The X or H pipe is in place to create an exhaust scavanging effect to draw spent exhaust gases out of the cylinders. Only problem is for the X or H pipe to work effeciently, it has to be relatively close to the exhaust manifold. On the Z, you can't get it close enough to the exhaust manifolds because of the transmission. So basically an X or H pipe will increase the asthetic appeal of your Z because it will sound cooler. No real HP gains.
