Ok,
Here is my reasoning on why the ARQRAY with its 3D Design quad carbon fiber diffuser is the best go-to bolt on solution for the 135i.
Top three
1st
ARQRAY - ~76cm tips spacing.
Diverted pipes, unvalved ~$2.5k
Why you shouldn't - Price. Some small trimming required of the underskrit next to battery.
Why you should - Looks great, perfect fitment, cheaper of the two ARQRAY systems. Bolt-on solution.
2nd
Eisenmann - ~76cm tips spacing.
Why you shouldn't bother - The side pipe is flimsy and is not a true quad. About the same price as the ARQRAY (>2K).
Why you should - Easy to get in Australia. Looks good and quality seems top notch. Compatible with 3D Design quad diffuser. Bolt-on solution
3rd
ARQRAY - ~76cm tips spacing.
Side pipes, valved ~$4k
Why you shouldn't - PRICE!
Why you should - Absolutely no trimming required compared to other system. Bolt-on solution.
Honorable mention:
-
SuperSprint - 58cm tips spacing
Why you shouldn't - Requires BMW 1M rear conversion. The Kerscher 1M rear bumper at the beginning of this post is spaced at 76mm apart so is incompatible. Given the price of their other products, this will cost an arm and a leg.
Why you should - You're planning a 1M rear end conversion and don't want to do the full battery tray / leads etc change too.
Other stuff;
- Remus - Looks like crap. Pass.
- HARTGE - Not sure on the tip spacing. Fairly sure it required the HARTE valence as well (which I think looks overdone).
- BASTUCK - If you google search some fittment pictures you will see that the tips sit about 15 cm too low and required their own specific valence which looks horrible. Given this, I'd avoid the Kerscher 1M replica rear end as well as they are using the BASTUCK exhaust on the pictures on the OP and this risks of not working with all the other stuff should you decide to use those exhausts instead.
Have fun guys.
EDIT - The only alternative is a full conversion ala Stu @ BMR
http://www.1addicts.com/forums/showthread.php?t=818172