The max speed quoted in various places,and dependent on engines, varies from about 260-275 mph,which equates to 420-430 kmph,or about 230-240 kts.That would be a max level speed at best altitude.
420 kts is about 485 mph or 780 kph,and personally I don`t think the perspex around the cockpit would take it.
I think we have to see if anyone has either a good readable cockpit photo,or the word from the Pilot`s Notes...