go with a full system, a slip-on won't do much at all maybe 2-3 hp... a full system can go anywhere from 6 to over 9+ hp.
If I where you I'd go for a full on HMF bill ballence pipe. It's not that loud, it performs, and it is definitely high quality.
Spark's pipes are not all that they are cracked up to be, so I personally would not go that route... their loud, heavy, and not the best pipe for 550 bucks...
Also stay away from Nmotion pipes... a whitebrothers makes more power, and is ALOT quieter. Nmotions are extremely loud pipes. more so then even the TC shorty.
when it comes to pipes, I definitely know mah chit, I've been doing research on the best performing pipes for over a year and a half now and I can say without a doubt the top pipes ever made by any company are.
in order, DASA, Rossier, HMF, pro circuit, CT, Whitebrothers, Nmotion, yosh, and lastly sparks x6....
I beleive, an HMF or pro-circuit would be your best bet since I know rossier and DASA do not make pipes for a z...
a TI pro-circuit will rock so will an HMF, and pro-circuit has it's own custom sound no ones ever reproduced, and I know HMF is also a great pipe for the money. go with those two, you can't loose.