I invite you to read reviews that actually measure efficiency. It isn't marketing crapola, it is fact. Hell, even read the 80 PLUS specifications.
http://en.wikipedia.org/wiki/80_PLUS#Efficiency_level_certifications
I invite you to ACTUALLY read what i posted.
I can summarize if you want:
Peak efficiency is between 50 and 60ish percent depending on the platform, not disagreeing with you there.. Say your rig uses 100w
max at full load (just for easy math). To get peak efficiency, you would need to get about 200w power supply.
BUT, this assumes that you are running your rig at 100% 24/7 (or whenever you use it). I really do not find this a likely scenario for any rig other than a render farm, F@H rig, or whatever. Most normal rigs sit at idle most of the time.
In the case of the sample, for simple math, we will say it idles at 50w.
Since you idle more than you run full load, it makes more sense to buy a power supply that is MOST efficient at idle.
In the case of the sample, a 110-120w power supply would probably be ideal.
(again, it is just a sample for easy math, but you can apply to concept to anything)
50% more than you need is a load of crap no need to buy a 1500w power supply for a 1kw rig. It's just silly.