Kailec
1. No 340mm don't look to small on 18" rims, but kind of with 20". (340mm rotor btw, the factory rotor size for some F3x like the 335i.)
2. Not all 18" rims will work with 370mm rotor and the according caliper.
Here is a list of all BMW rims, that are fine with the 370mm rotor.
If you have any aftermarket 18" rims, you need to check if they free the caliper if using the 370mm rotor.
http://www.realoem.com/bmw/enUS/show...diagId=03_0108
3. You don't see that listed because the US version 328i S2NHA option only upgrades to 340mm rotor and the "small" blue caliper in front. (In Canada you get the US version as well, I think. Check with your VIN Code)
Every F3x will get 370mm and "big" caliper, with the M Performance Brake (red, yellow, orange)
Every F3x 335i/340i will get 370mm and "big" caliper with the S2NHA Option (blue caliper)
On every F3x you can, from a technical point of view, mount the "big" blue S2NHA caliper of an 335i/340i.
If you want "big" blue caliper and 370mm front rotor, just look at the front part of the 335i/340i. S2NHA=YES
If you want "small" blue caliper and 340mm front rotor, just look at the front part of the US model 328i. S2NHA=YES
For the rear parts take exactly the one of the S2NHA for YOUR car (because of the rear brake rotor can vary due to emergency brake differences of F3x models)
http://www.realoem.com/bmw/enUS/show...diagId=34_1970
Perhaps you also look at my F3x brake thread.
http://f30.bimmerpost.com/forums/sho...d.php?t=858607