From what I have read and looking underneath my car and also having dealt with exhaust upgrades & Cats with my last car - My guess is its this.
1. The PE exhaust diameter might be different for my E90 exhaust Pipe - as BMROX said - This is as simple as getting exhaust shop to to flange it out to meet the change in diameter
2. The US version: 18100443581 - Replaces the centre pipe with a straght through pipe. If you have a car thats 2007+ you would have 2 CAT's very close to your dump pipe when the exhuast comes off the engine (which is why there is no cat near the muffler). If you have a E90 that is older than 2007 - The CAT will be located where the muffler is. The main issue with this is if you change the centre pipe and take out the CAT - your car is 100% illegal and there are serious fines for not running a CAT. You could simply cut this centre pip in half and put in a high flow cat (problem solved!)
BMWminiparts are doing a great job at finding some of this stuff out. In combination with me measuring my pipe diameter we should have a resolution.
Have gone through all this research - it just seems to me like the logical choice would be a Eisenmann Sport Muffler 325i E90 or Race Muffler - This is said to be 100% campatible and Cheaper!
I am not sure if the Race Muffler would be legal from a EPA sound perspective - Apparently it is 93DB - In VIC i am not sure if the sound limit is 90Db or 96Db.
Hope this information has provided some help for others out there
- any advice or info keep me posted!