I certainly wouldn't argue against the Seasonic G-550 (for some reason I thought the link was to an EVGA 550, a CTW or FSP OEM). I would expect a Seasonic to manage the rated power 24/7 for the duration of the warranty.
In any case, the point I make about efficiency was primarily a suggestion, not a strong diversion from the 550 or certainly not from Seasonic.
However, efficiency and heat go together, and heat and deterioration do too. If you look at some of the reviews, for example, you'll notice support for your point...the difference in efficiency on the 550 from 300 watts to about 450 watts is, as you pointed out, only about 1%.
The heat from exhaust goes up about 4C in that range. This is really what I'm getting at. I tend to assume this kind of thing is obvious, but I realize it's not. A 1% change in efficiency isn't going to change one's power bill by a useful amount, but a 4C increase in the heat from the exhaust indicates how much harder the unit is working.
If I run a machine 24/7 (I know, the OP didn't say), I tend to consider how that power supply will perform in 4 or 5 years. For some office machine running 8 to 10 hours, I don't bother.
I consider that point because I frequently end up with a "retired" machine that's 3 or 4 years old, and a PSU that might be applicable to other builds if, when I bought it, I chose something I knew wasn't going to be used up in 4 years. If I do elect to use an older PSU, I want to know it would be useful for at least another 2 close to it's operating spec. One of the ways that does NOT happen is if the unit was used at or above 75% of it's power rating, where everything was hotter all day/night, and .... here's the part that matters most to me ... after unobserved lint has clogged heatsinks, fans and openings.
I agree it's not a reason to spend $35 on a build where the budget matters.
For an old engineer that insists the PSU is the last thing that will ever give me a problem, I have certain margins I keep in mind which ensure that.
The best builders, Seasonic among them, choose to label a unit at, say 550w, when the design itself, if reverse engineered, would actually qualify to be a fairly good 650. So, it's likely the G-550 is already beefed up to account for the point I'm making.
That said, there are some Superflower builds which are similar efficiency, at 750W, at the same price as this 550, which survive their hotbox tests with superb results. The surprising CAPSTONE 750, for example...I would never have thought I'd suggest a Rosewill PSU to anyone, ever...but it's a Superflower which reviews show to be a very good alternative option at the same price point.
Don't take that as suggestion to consider over the Seasonic. The one thing about Seasonic is...they're one of the very few that actually make power supplies. That's a big deal!