For what it's worth, both of these are probably normal. My tailpipe slings soot on the back bumper, though I have mineral grey so it doesn't really show.
And out of the 3 BMWs I've had (E46, E90, F30), each one has had a speedo that trends high. If I'm doing 45 per the speedo my iPhone GPS will indicate 43, or thereabouts. I guess it could get up to a 6 MPH delta but you'd have to be going pretty fast.
2012 328i Sedan, Mineral Grey / Black Leather / Aluminum+Red Trim, 6MT, Sport Line, Premium, Premium Audio, Tech, Apps, Assist, Xenons, Folding Seats