Nah, they can only void your warranty in total if the car meets a few specific criteria like odo tampering, declared a total loss by insurance, etc. Normally what you will face is a denial of a particular repair on a case by case basis or having your powertrain warranty voided because of power adding modifications that (they can say) put stresses on the drivetrain that it was not designed for and changed it into something that they did not sell you. No one is obligated to warranty something they did not sell.
Dont even bother with the idiotic "the dealer has to prove it" crap because you are not in possession of your warranty, BMW is in possession of your warranty and they can deny you whenever they want. The way it goes down is that you bring your car in, they spy those shiny new test pipes (for example) where your primaries used to be but the dealership does nothing, says nothing. Most dealers would rather just service the car and collect. In other words they are mod friendly or at least forgiving because it costs them to be otherwise. So that power mirror, trim piece that fell off, even miscellaneous drivetrain issues get fixed and nothing is ever said.
I said miscellaneous drivetrain but there is a kicker. BMW will not authorize any repairs for a whole bunch of expensive parts until a regional rep comes in and inspects the car. This guy is not mod friendly, he doesn't sell cars, you are not his customer. It is his job to look for those power adders and he is well trained and has no mercy. If he finds them he will deny your warranty repair at least on that case and depending on what he found, possible trigger a voiding of your powertrain warranty. At that point your dealership cant make the repair unless its willing to buy the parts and provide labor gratis. Most of us are not going to be that good a customer. This is where the "they gotta prove it" thing falls flat on its face. They don't have to prove jack. At the time of denial you are either coming out of pocket or leaving the car broke until such time as you go through a very lengthy process and do just exactly the opposite, prove they were wrong.
In practice for most of the parts that we install you will never have an issue getting your warranty work done. Even with power adding parts as long as you choose wisely from well sorted parts sold by reputable shops you will still likely not face any denial of coverage if for no other reason that its unlikely your going to have a major failure that brings in that regional rep. This one isn't about getting lucky, its about what happens if you're that one guy that gets unlucky. Thats where you have to ask yourself, am I financially capable and prepared to be my own warranty station if the worse should come to pass?
In life I know of no one that was ever given any type of grief with a cat back. They don't add any real power, and effect essentially nothing. In terms of emissions they are specifically allowed by the EPA as non emissions sensitive devices and it says just that on their website.
2012 E92 Space Grey with fox red/black/black, comp, premium, DCT, premium sound, sirius, heated seats.