Full stress is practically meaningless when trying to factor in spikes, which happen on ALL hardware regardless of how good the rectification and regulation are. Just the GTX 1060 has been seen to spike at at least 140w. Regardless of any other consideration, it is not a best practice to include a power supply that is beyond the 60% envelope when it comes to the requirements of the system.
Yes, of course it will work. Will your power supply last as long as it would if you had a higher capacity unit that never exceeded 60% of what it could generally sustain for long periods at 40-50°C. No, it won't.
Will it be as quiet as it could be if you were only running at no more than 60% of capacity, at least on average? Nope.
Will it product higher ripple, higher electrical noise levels, have more uncertain and variable voltage regulation above that marker? Yes, yes it will.
So yeah, it CAN run perfectly fine, for a while, with a 550w unit. Perhaps a long time even. But if there is an option to gain some additional overhead without dramatically affecting the purchase price, which in this case there is, or if you can get a unit that has quality which is more than good enough for the average enthusiast build, but has a nice 40 or even 50% overhead, for the same price that a higher quality but lower capacity unit would offer, then I think you really have to decide whether efficiency is really the more important factor and whether the platform's performance is that much better that it makes it worth sacrificing these other considerations.
Of course, that's just my thought on the matter. A lot of people put a great deal of stock into what JG thinks on all thinks PSU, so I'll just add this little tidbit from him which happens to be agreeable in this case.
For a single graphics card, ANY single graphics card, that 700W is way more than enough. You could get along with a 550W even. But should someone, anyone, be telling YOU that YOU made a bad decision because the power supply is "too much"? No. There's really little to no disadvantage to buying a "too large" PSU outside of cost and who is some forum punk to tell you how to spend your money? Anyone that calls someone out for buying an "oversized" power supply is just being a snotty little know-it-all, and those that are telling people that they only need 380W power supplies will likely have to buy replacements for their burned out power supplies in a year or two time.
Often times, buying an "oversized" PSU is a good choice due to it's ability to perform even under higher temperatures, a quieter fan because the unit is delivering a lower percentage of it's total capability and considerable system upgradability without having to worry about having enough power when upgrading a graphics card, adding a graphics card for SLI or additional hard drives.
But if you really have to go with the absolute least expensive option that doesn't leave you with a dangerous product, then I'd totally agree that there are 550w units out there that can handle that configuration. Will it be as optimal as even a 650w unit that has otherwise good quality? No. Will it be acceptable? Probably, BUT it will also probably not last nearly as long AND depending on the unit, you may quickly grow tired of the additional PSU fan noise if it's a model with an aggressive profile.
Another thing to consider is that as time goes on, both AMD and Nvidia have consistently shown their cards to pull significantly more power after about a year or so. Driver updates and other factors are contributors to this, so basing power decisions strictly off the reviews that were first done when a given series was new, is not always the best idea. Recently we had a moderator whose card originally ran fine on a 550w Prime Ultra PSU, that after about a year required him to upgrade by about 150w due to increased power consumption resulting from changes in the driver framework.
His card was tripping the OPP of a perfectly good, nothing wrong with it power supply that simply didn't have enough capacity anymore.
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