Hmm, interesting. I've always oversized by PSU's as a matter of course, since I've always thought working at 60% capacity is better than 90% or whatever.
I usually drop a 750 watt 80+ Gold into most of my builds, even though a 500 watt or even a 450 watt would be sufficient with a single GPU, and have no plans for a second GPU.
aiming for 50-60% capacity during typical operation is the standard recommendation.
Efficiency usually starts tapering off below 50% and below 30% it falls off a cliff - however, that just means instead of an ideal 10W power consumption you're actually pulling 30W or something like that, it is usually not a big deal in absolute terms.
(there are also some exceptions, some of the platium/titanium PSUs actually can hold pretty decent efficiencies right down into the basement.)
750W is a good "standard" recommendation, that's enough for any one GPU on the market.
The rule of thumb is really more to guide people not to buy 1600W or 2000W monster PSUs just because "bigger number is better!".
(Although those giant PSUs do have the advantage that they can often run completely passively under load, they won't kick fans on until 50% or 60% load, which for a 1600W PSU means you can comfortably run a high-end GPU and a high-end CPU completely passively.)
The selection was "gold", specifically. And that's not as bad as it might be, but titanium is better across the board and much better at low load. A titanium supply is more efficient at 20% load than a gold supply at 50%, for instance.
If you're over-sizing your power supply by ~60% (as is the case here) then this is significant.
I'll keep that in mind on my next build. The pricing steps up quire radically though, it seems.
But, you build enough "rigs", you learn not to skimp on certain components like PSU's, Cases and Motherboards... which is normally where new builders cut corners.