+1 for midpipe + valve controller. You definitely want the valve controller as a mid pipe on it's own can be a bit underwhelming.
It's not likely to impact warranty and remember that it's actually in a dealership's best interest to do warranty work. They get paid from BMW HQ for the work and help strengthen a relationship with a potential future customer. Engine tuning etc. is a different story because the computers can detect it, but small hardware mods like an intake and exhaust tend not to have any impact.
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