When a PSU temperature goes up, the efficiency goes down (the mobo asks for 200W from the PSU but in order to get 200W to the mobo, the PSU must draw 300W ... now we factor in RMS to get a wastage of 12W .... the more efficient a PSU is, the less wastage). Now about the temps, the more efficient a PSU is to start with, the better chance of it not failing you (not delivering power/voltage stability) when the temps go up (and they will)