Hm, the hp is absolutly ok, the loss of power above 5500rpm mey have several reasons:
- it could be, that the dyno-room doesn't brings enough fresh air to the car. The car gets hot than, the air doesn't have enough oxygen for a full ignition.
- it could be, that the ignition-plugs are not clean. Especially outside Germany the ignition-plugs have often the problem, that they can't burn free. It is could for the engine, to get sometimes high-speed-trips on the "Autobahn" for full power. It might be, that your car has no problems on the dyno anymore, after to 200km-trip through german motorways.
- maybe the qualitiy of the fuel ist not could enugh for full power above 5500rpm. Especially at high rpms is that important.
Questition: do you feel that problem driving on the street as well?