Grand Ams are quick for their class of car, but you're not gonna have any problems with them....
As far as your top speed, who knows. When you say your speedo is off, do you mean offset b/c of the gears? Drive beside a friend and see how much off (maybe about 10 mph--- eg. when your speedo says 65, you're really going 55). So then, how fast where you going at the top of 4rth??? Perhaps 110? I don't see 140 mph... on a looooooooooooong stretch of road being too difficult.
I'm just guessing....