You really do need to actually check that you are getting wheel sensor values though - on my E46, nav was dealer retrofit, and it had one of the faulty factory-made cable looms with the wheel sensor pin in the wrong receptacle on the plug into the nav computer.

Gave the same symptoms that it would never be accurate when approaching junctions, turn-offs etc. When it was fixed, the nav accuracy was spot on.

Check you get a reading when moving under the test menu (Wheel Sensor) and if not, probably a trip to the dealer