Increasing the weight of blades reduce efficiency since input power Pin=T.W

where W is angular velocity.

so if Pin increase, efficiency decease.

In a steady state it is none! But wind is variable so that the higher the weight the higher inertia and more energy will be invested in acceleration and this is a loss. This means that -even if you neglect friction loss due to weight- the rotor will accelerate less in following wind speed variations.

Generators in wind turbines can be either designed for variable or constant angular velocity but variable angular velocity generators cost more.For a periodic wind velocity , inertia of blades act as a flywheel and reduces angular velocity changes.That's why for wind turbines with constant angular velocity generators,It's better to have heavier blades.