If a generator lights a lamp, the energy from the mechanical source turning the gen lights the lamp, plus a little energy in the ineffeciency of the generator, it makes little difference whether the lamp is 3v or 300v, the amps x the volts is the same for any 2 lamps of the same wattage, and the torque on the gen the same(so the amount it slows down the same).
If you used a conductor with higher resistance in the generators windings unforetunately the efficicency would go down, as energy would be lost as heat in the resistance of the windings.
I dont think it will be that easy to find a new trick way of generating power though it may be possible.
To answer your question of effeciently reducing current, that can be done in 2 ways, with dc you can use pwm, and ac is simpler you could just use a choke, allthough both methods would introduce small losses.