Porsche claims 550 hp for the 971 Turbo. Mfg horsepower claims are always "engine stand-dyno horsepower. Sometimes mfg understate horsepower. Example: the famous L88 GM motor from the 70s.
RaceChip dynoed the bone stock 971 Turbo at about 550 wheel horsepower. I think other tuners have had aprox the same wheel hp dynos.
Assuming about a 15% power drive train power loss the 971 Turbo probably makes about 647hp on an engine stand dyno.

 

Porsche has historically almost always published the HP figures for their lowest run, not the highest. That way you will see at least published figures, I suspect most see more.

 

There has always been "Porsche Horsepower" and everyone else's. Maybe the rear wheel measurement versus crankshaft 'splains it.