I achieve top speed in my 996 cab with the open top, 286 kmh on the digital speedometer and Seven is right, with the windshot there is minimal turbulence in the cockpit, it's very impressive but the noise is just ridiculou, obviously, actually, above 150 kmh, the noise pretty much makes you have to shout to have a conversation, it's a good speed limiter!
But it's very tiring on the motorway (because of wind noise) and in city centers (because of car noises and fumes), bliss on country roads!