What is the basis for this 'sweetspot' of yours? 50% load is almost always more efficient than 70-80% in 80 Plus certified PSUs and more current = more wear and tear. Given that the current / power that is being utilized is the same regardless of PSU (other than differences in efficiency,) the 'stress' on the PSU is basically the same. Because higher-wattage PSUs of the same platform typically use more/bigger caps, transformers, coils, etc. the 'stress' on each component will be lower.