this might help you sort out what you want done ,
http://www.scorpion-exhausts.com/Cars/s ... ?model=142
the scorpion exhaust for the Z is the SAME diameter as the oem exhaust so 0 gains there , the aftermarket Y pipes , motordyne XYZ , japspeed , envy etc etc are 3inch and have much bigger more open colection pipes from the cats.
i had my y back fitted a few months back wiht bascily just a noise improvement , got Y done this morning and can say its either the extra noise or it is maing a difference not huge but its seems better.
if you just want power get a cheapish FULL exhaust Y pipe and all liek the K2/K1+ y etc but unless you do the full thing its not going to be notuiceble then get it mapped . also thew cats if you ever get chance to look at them are feable !
(note if anyone feels im wrong here please dont hold back )