so what i've been told with 130i owners, 125i and 130i is the same.
so from the link from babybmw and these photos from a friend, you could diy the pe exhaust.
also, if you have an auto, like me, you cannot Y pipe the resonator. I have done the golf tee mod and went back to stock because there was a slight drone and this is on a stock exhaust. my plan is to only do the back box to this pe style mod. Leave the sound insulation in and should sound a bit louder when wanted.