A minor update for those interested in this subject.
I decided to upgrade one of my favorite computers with one of these power supplies. It is my old gaming system from 2003... Abit NF7-S 2.0, Athlon XP 1700+ (was at 2.1Ghz but the CMOS battery died and I can't remember the settings!), 2GB DDR-400, 7600GS AGP 8x (later addition), etc.
With the old-school, totally inefficient (and with almost no certifications on it) Antec PP-303XP 300W PSU, it was pulling around 131W from the wall while idling at the desktop. Despite nearly doubling the rated capacity of the power supply by installing a 550W Seasonic, it still managed to drop the idle power consumption a few watts to around 128. This doesn't seem like a big deal, but it should give others an idea of what to expect when using a hugely over powered PSU on an older system. The upgrade in efficiency seems to win out over the drop in percentage of system load (around 43% load on the Antec 300W, to around 23% load with the Seasonic 550W).
So, don't be too concerned about losses in efficiency. To get that solid 30A on the +5v rail, its certainly worth it over just using an old, likely less reliable model.
Now for some blitting from the back buffer.