The enemy of speed is aerodynamics.
When the wind resistance balances power, you hit the wall.
Wind resistance increases with the cube of the speed, so you double your speed, you triple your wind resistance.
Even stock, the final drive ratio was too tall on the RC51.
By reducing the final drive ratio, the engine was able to operate in its optimal power band.
But that also reduces its mathematical top speed.
On the plus side, the RC51 was quite aerodynamic amongst Honda product line of the time.
Even with less HP, it was able to attain the same speed as the CBRs due to less wind resistance.