There is a practical limit to forward speed for single rotor helicopters which is the ratio of tip speed to forward speed called the advance ratio.
There are limiting factors on both the advancing side (sonic issues) and the retreating side (low V squared, high AoA and stall).
Advanced blade designs can operate at higher AoA on the retreating side and higher speeds on the advancing side.
Having a lower tip speed due to a shorter rotor helps on the advancing side but you would have to increase the rotation speed to keep the mass flow the same so it's not a free lunch.
Ray Prouty used to say about helicopter design 'What you gain in the hover you lose in forward flight and vice versa'.
One other aspect of increasing engine power in a twin is that it allows PC 1 profiles at higher AUM so more bums on seats in the cabin.