I think Craig has it right, the sports group is limited by the rev limiter which means you can go above 140 mph under certain condition (ie, downhill, wind on your back, light loads etc) but not much faster because you can't beat the law of physics. With the non-sports package cougar, you have a speed limiter which is there to save your life due to the sh!tty tire Ford put on the standard cougar.
someone once caluclated the theoritical top speed of cougar based on the final gear ratio of the car and MTX is something like 155 mph and ATX is close to 170 mph. But at last, Mr Newton made sure we can never go that fast.