An unmodified USA model would hit the low 150's.
fast
You can try googleing "timing belt replacement on porsche 928". When replacing you start at the crank gear, go under the oil pump gear, up around the drivers side cam gear, under the water pump pulley, up over the passenger side cam gear and then down thru the tensioner and back to the crank gear. The tensioner pulleys go on after the belt is routed.
Around 160mph
Around 170mph
About 165 mph
209mph
Around 160mph
20 MPH
160
they go quite fast and therefore are the cause of a ton of accidents
My 06' Carrera goes up to 178mph
bout 250 to 300 kmh