I've had my injen intake for about 15k miles now. I've been to dealership so many times and it has never been mentioned for warranty. This includes when I get my injectors replaces because of engine ticking and also when they had to replace my AC system and my car was with them for a long time. If anything the technician was super nice and said I had one of the coolest 128i cause those are almost never moded.
As long as you the install correctly you have nothing to worry about. Now, if you drive into a lake and your engine dies because the intake sucked in water, better get your wallet open xD
2009 128i Coupe Crimson Red
MODS: -Injen Intake-Magnaflow Exhaust-Carbon Fiber Spoiler - Carbon Fiber Grilles - OZ Ultraleggeras - CF Mirrors