There are improvements in drag, but power hasn't really changed, it can't tbh as the rider is the one creating it.
this isn't really true - is it?
My thinking was that for any given RPM the mechanical resistance of a generator is proportional to the power requirements of the load (bulb).
So if you switch from Halogen to LED you can get the same Light output in Lumen for less power input since the LED is more efficient. This means the Torque required to turn the generator will be less and the mechanical resistance reduced.
Of course you could have a brighter light for the same torque instead if you want - which is probably the case with most new dynamos (im guessing as Iv never had one).