The N54 mids are more restrictive than the N55 mids. Which is why a lot of N54 guys try to find N55 guys that just upgraded to a full system and get their mids for cheap.
I have never come across an FI car that DIDNT come with a restrictive DP from the factory. As for the axle back portion im not sure. Cant see much difference from the 135i to the 335i in that specific section.
As for dealer and warranty. Find a dealer that is mod friendly. Yes, they are out there. Get in good with a SA and talk to them about what you intend to do. Find one that will be real with you and not just be the annoying asshats that say "any mods void warranty...." because they are wrong and just being annoying asshats.
If you are THAT worried about it then either stick with BMW Performance parts or get parts and learn to install them yourself so that you can swap them out when it comes time for a service visit.