Gilles, see if you can do it on a Dyno Dynamics machine in the Porsche shootout mode (P1) for best accurate results.

I know some people like Mustang dynos and Dynojet but the tuner said they all read high. He called them "Hollywood Dynos". Well that was his comment, valid or not beats me.

Go here to find a Dyno Dynamics machine in your area:
Dyno Dynamics - Dealer Locator