Providing less power = using less power. Its better to be sound that incompetant with PSUs. But this 300W is not using too much power.
Looking at some PSU reviews on Tomshardware recently it seems that they are actually more efficient at full load than a medium load and with a light load the efficiency drops off quite a bit with some.
Have a looksee here http://tomshardware.co.uk/2006/02/07/5_power_supplies_get_the_full_juice_treatment_uk/page15.html#efficiency_ratings