400w DC or AC? If it's 400w AC, the actual DC power draw is lower. Also, the typical workload (rather than having the CPU/GPU running at 100%) may be under 300w. So he may achieve the best efficiency at full load; however, it will worse during his idle and typical load thus making him pay more.
Either way, the difference in efficiency isn't that significant. Other than that, there isn't really a negative other than the fact that the higher wattage of the same series will cost more (with the exception of sales and such). There's also an issue that may relate to audible noise from the fan, as a larger capacity PSU may require a faster fan to cool the amount of heat that will be dissipated.