If you're gonna upgrade something then you should UPGRADE to something truly better. Some people have said the Stillen rotors are good but they aren't the best. Same goes for Z1. Now I don't mean to diss either brand. They're both reliable and make awesome products. But if your going to upgrade your rotors I say get some 2 piece rotors. DBAs have good rep. Z1 has some 2 piece rotors that are track tested. CZP just made some 2 piece rotors. Just do your research
btw Z1 has a sweet upgrade sport package brake kit for a decent price