If you add more horsepower you will be able to run a taller gear, or larger diameter tires and then you will have a higher top speed. If your car can already hit redline in top gear, then more rpm capability is the only way to go faster without changing your gear ratio. If your car hits a point in top gear where it just won't go any faster and you're turning some rpm less than your redline, then more horsepower will increase your top speed. This was the case in the ZR1 Corvette, where in order to hit top speed it was left in 5th gear which would run about 180mph, then 6th gear was too tall and the rpm would drop below it's peak horsepower rpm and the car would slow down a little. In that case there was room for more horsepower to improve top speed. The McLaren F1 redlines in top gear so the only way to go faster is make the final gear ratio taller.