The bad thing amongst all these internet dyno horsepower wars is that people dont do enough research. Back in the day turbo imports became dyno queens and Euro cars wanting to follow that trend started using wrong dynoes (Dynojets) in order to make a fake impression of a higher hp gain. Back then most Euro cars were naturally aspirated and for the part companies it made sense to test their product on dynojets. The true wheel hp that is being put down to the black is displayed by dynoes like Maha or Mustang dyno. The OP's numbers are most likely as close to reality as it will get.
Current: 13 M5; 02 M5, 14 X5, 06 M3, 05 M3, 15 I3.