Not in the Boxster of course, but I once hit an indicated 170 mph, 3/4 of the way down the 3,500' main straightaway at Miller Motorsports Park.
Legally ! Then I had to brake, hard! I was on an MV Agusta F4 1000s (motorcycle). Top speed is supposedly 186 mph (300 kph). The world seems to go by really quickly at that speed, not protected by a cage (car).