At the risk of starting a huge war... I've noticed that whatever kind of dynamometer tends to be widely available in Europe always seem to read high, and the estimates they provide should be interpreted as crank hp NOT whp.
Also, Pferdestarke (PS) units are a lil bit higher than hp...
see further here
Power Units Conversion Calculator
On a dynojet that car would be putting down around 288 - 290, SAE corrected, which is about right for a nice exhaust on an otherwise stock Z.