The trend for hardware now is efficiency. In a couple years he'll be upgrading to components that use less power.
True, efficiency is the aim, but ID would need to add just one of these 'efficient' components and he's underpowered, based on what he already has.
Also, have you considered multiple HDDs in your evaluation? What say he has 6 x2Tb drives; a sound card; a TV tuner and a couple of SATA/USB expansion cards to add from his current machine?
Nothing like that was mention because ID was referring to the new compnents, but nevertheless, whether he has all these extras or not, the advice still stands. It is better to be slightly overpowered than to not have enough, as you're advocating. And nobody is suggesting that ID go for a 1200w or a 1500w PSU. A 750w to 800w is quite sufficient and certainly not overkill, as it seems you are suggesting.
Put another way, 95% of the streets surrounding you are 35mph/60kmph, therefore I recommend you purchase a car that can do only 36mph/62kmph, no more. You'd argue that you'd need more power to get yourself out of trouble at times, and I'd argue it isn't necessary, given the prevailing speed limit and road conditions.