Since Option 840 "Increased speed limiter" is simply a bit of code in the engine processor, it should be a trivial change, if you can show a dealer that the car has the appropriate tires. It probably means reloading the full engine software. A full reload of all the vehicle's code can take a few hours, so expect to pay accordingly.
If you do take your NA spec X3 to Germany, it's probably something that's best attempted there.
If you can't find a dealer to do it, you might want to speak with someone from Dinan (in the US) or one of the German BMW tuners like AC Schnitzer, Koenig or Alpina who undoubtedly know how to remove or modify the speed limiter.