The spec'd RM for a P90D should be 270, right? Sounds like you got a good pack. I don't have a 90, so have no comparisons to offer, but keep in mind that there are plenty of factors that will cause variations in the RM shown on the display (IMHO, none of which have anything to do with how you drive the car). The two biggest factors seem to be temperature and charging habits, which throws off the battery's calibration.
Be glad that your number is as high as it is, but don't sweat it as those max numbers will drop.