I reason you want headroon in you psu is when psu start maxing out efficiency falls quickly. when efficiency less current is produced and more heat is putout. As temps increase efficiency drops even more. Leading to a burn out psu. I you know your system draws 280 watts max. Look for a psu with a output of 500 to 600 watts. Psus are most efficient running at half power, but you need to get a high quality psu too. There are no standards use in rating psu. Take a good psu vender like Corsair they have more than one 750 watt psu,but they are not equal. The cheaper one are rated at a cooler ambient temp but the top of the line psu are rated at a stated 50C. A psu can lose a lot of output as ambient temp rise. As much as 40%.
That make absolute sense! I didn't even think about efficiency, good thing I have my hx750.