My E30 was sloped up to an extra 6-8 km at a 100 then back down to be dead on at 200 km/hr. This may or may not be the same with the E53, I think some sluthing and testing may be in order!
Back in the day the dealer told me that the speedo was calibrated to be accurate at 200 km and as long as it was within 10% in between there was no problem.