if you upgraded to a low end 700w vs a high end 500w, you would be losing more than what you will be gaining
anyways, efficiency of the unit is determined by how much power you are drawing. power supplies tend to be most efficient when the load is around 40-60% of the rated output of the unit
therefore for a 500w, assuming it can output 500w on the 12v rail, will be most efficient when the system is drawing 250w
for a 700w, assuming it can supply 700w on the 12v rail, it would be most efficient at around 350w.
if you are not hitting those marks, efficiency will drop, therefore consuming a bit more power