Top speed? I have had the car to 120 on the track at IRP. We know from experience that the V8 SHO in stock form is good for 140+ (I think 144 is the highest documented at this time, from several people). The car is slick to the air and makes good top end hp. To get those last few MPH takes some distance, but the car is good for it given safe running room. For sure the 150 mph speedometer is not there just for decoration.
Check out: http://www.v8sho.com
There is a picture on the main page of one person's run!
Of course proper tires and safety equipment are highly recommended and I do NOT recommend anyone ever try this on public roads other than those officially shut down like the Silver State etc.