Depends entirely on the brand. LC Power for instance will explode if you draw 550 watts from their 550watt powser supply (I know), an XFX 550 watt will sweat and make some noise but still provide you with 550 watts.
Efficiency relates to input power versus output power. This is what the 80Plus standard is designed to value and represent.
Different power supplies will have widely varying efficiencies, and even those that meet an 80Plus standard will still vary to some degree at different load levels.
The basic 80Plus rating means that at 20%, 50% and 100% load, the PSU achieves an efficiency of at least 80%. The subsequent Bronze, Silver, Gold....etc represent increases in this value.