I'll take a stab at this and say "a little." The potential energy you gain from climbing the hill = mass * gravity * change in height. Going down the hill converts that potential energy to kinetic energy (0.5 * mass * speed^2) but the faster you use it up (i.e. higher speed) the more is lost to wind resistance (doubling your speed = 4 times as much wind resistance). Basically, going down the hill faster will be a less efficient use of the energy you stored when you pedaled up the hill.