And when a Gold model's warranty is two years longer than the equivalent Bronze model's? Is that just the company trying to fool us into thinking it's better as well (it might sound like I'm being sardonic, but I'm not - it's a serious question)?
There is no absolute rule in purchasing a PSU that will guarantee avoiding failures. But yes, many bits of information can be pieced together to get a decent approximation of quality if you don't have access to a teardown review of the unit from sites like JonnyGURU or Hardware Secrets.
I will point out the old caveat of statistics: correlation does not imply causation. Or in this case "full causation". The 80 Plus cert doesn't check for what caps are used, what protections are present, such as short circuit, over-voltage, etc. So long as you meet the requirements, you get the cert. Yes, there is some "bare minimum" to get the cert, but this minimum is not that "high" as one might think. Any further information cannot be gleaned from the cert itself, but yet another variable, such as warranty, as you mentioned, brand recognition, such as Seasonic, or the more typical "store review" at Newegg or elsewhere(those store reviews aren't usually useful, but "special case" reviews can be useful.
They can't even control what units the label is slapped on, as Raidmax sent in a "Gold" unit and then put the labels they acquired on some Bronze units they had. Of course, not every company would do such a sleight-of-hand, but the cert is vulnerable to these tricks.
http://www.hardocp.com/article/2011/10/04/80_plus_irrelevant_to_you_when_buying_psu/#.UZWbNqLVCSp
Now, warranty length is a separate variable that can be used to "predict" PSU quality in case you don't have a teardown review and/or electrical engineering know-how to understand fully what you see inside the PSU.
You can use warranty and the 80 Plus cert "together" to get a feel for how probable the unit is a dud or quality unit. But now the subject has changed to using warranty and the 80 Plus cert to determine likeliness of quality, not the 80 Plus cert alone
If you want the statement of 80 Plus Gold>"lesser units", as-is with no hidden qualifiers, to be consistent, then it must follow that snatching an 80 Plus Gold unit from any company, including suspect ones from Coolmax or Raidmax(Raidmax as a Gold unit with only 2 years warranty), will assure me, that they will "perform" better than some "lesser" certified unit from an older generation, or even a non-certified one such as the Antec VP-450. Of course, there will always be some DOAs, but quality control can reduce the percentage of duds and to-be duds.
The statement may not hold in the absolute sense, but you can use it to get a feel for the probability of avoiding a "dud", as there is some correlation between higher level certs and reliability and/or performance.
I would say warranty length is a stronger indicator of quality than the 80 Plus certification, but use of that measure does have some limitations when in the territory of 5 year vs 7 year ones, as many units with 5 year warranties are good performers.