Would increasing rotor disc diameter increase efficiency in a helicopter by reducing disc loading?
Yes, increasing rotor diameter increases efficiency by reducing disk loading, exactly as you say.

From J. Gordon Leishman, Principles of Helicopter Aerodynamics, section 2.3:


  ..vertical lift aircraft that have a low effective disk loading will require relatively low power per unit of thrust produced..and will tend to be more efficient; that is, the rotor will require less power (and consume less fuel) to generate any given amount of thrust.