The change in efficiency depends on a number of factors.
In pure mechanical terms there is no difference in cycling at 200watts using 170 or 177.5 cranks. Longer cranks do not generate power, only your legs do that.
Longer cranks have a larger turning circle and each rev is more work so you generate more torque and can turn a larger gear (with pedalling force as a constant)..You deliver your power turning bigger circles more slowly.

People with longer legs should really use longer cranks.
For any one person there is a range of suitable sizes, usually +- 5mm from your "normal".