Either the 12.75 or 13" will have a noticeable affect, but If I'm not mistaken with the Z1 13" kit now you have to buy rotors from z1 if and when the time comes to replace your rotors. With the 12.75" kit the rotors maybe more locally available and then your not backed into a corner having to pay z1's price for there 13" rotors and wait for them. I could be wrong about all the above, but all the above is what has held me back from doing the 12.75" or 13" z1 kit. So if I am wrong please let me know.