Obviously there is many factors to take into consideration like wind speed, resistance, aero, gear ratios etc etc but just say hypothetically speaking you have X car with X resistance, X speed wind, X aero etc etc, with X amount of power. it goes X top speed now you decided you want your top speed to be faster. So you build X car to have Y amount of horsepower. Now you put it in the same exact conditions as before and all parameters are exactly the same except for the amount of power. Does your top speed still stay at X just gets their faster? or is your top speed now y? to me it seems like horsepower and torque only decrease the time it take to reach top speed and not actually increase the top speed. anyone care to explain?