Also, I'm very keen to hear the sound of miltek exhaust too, but there isn't any video regard it on their youtube channel, and their product has same tailpipe size of regular 118i but it should sound more sportier.
You can check some of Milltek product for inline 4 engines on their channel if you're interested how some of them sounds.
Here is the cat-back product from their site and you will see that it's single tailpipe solution:
http://www.millteksport.com/exhaust....5#.U2nvS_mSw_Q
The only compatible for all F20 series is their most expensive and latest one Racechip TB Ultimate.
It was also very important to me that it wont affect the warranty and it doesn't, but before taking you car to your dealership for regular service or anything else, you will have to remove the chip installation by yourself.
But don't be afraid as I'm not car mechanic and I didn't even change a tire in my life, but I made to install it by myself.
Only problematic part of installation was to reach the boost pressure sensor which was below the intake box + I have a huge arms on top of that, but I made it in the end with little bit of stress and sweat.
Product itself comes with warranty, but it says "Made in Germany" so that gave me some confidence.
ACS has to be installed by official BMW dealership or by ACS itself what I heard.
If you decide to go with Racechip or any other product, you can freely ping me on private, as only part that wasn't mentioned in this product installation was where to safely store this chip, and not cause any vibration under the hood and keep it away from engine as much as possible. I can send you pictures where I have put it and save you at least half an hours of hard thinking where to stick it.
All the best!