Not how it works, Super Flower, Seasonic, Corsair or anything else are not 'good PSU', because these are brands. Their specific models can be good, or not so much, or straight up bad, depending on what you look at. You can't just assume that just because the brand has some good models - that the rest would be just as good.
He stopped at Leadex III because, yes, it's a older post first, but second, which is more important - there are no reviews on this model.
I don't know for sure that it's bad, but i don't know if it's good either. But there's a review on Leadex V Pro Platinum, and it's not so good. 103% OPP, meaning it has basically no power delivery margin whereas it's normal for this kind of PSU to have 20-30% margin, meaning that compared to competition, this 850W PSU you've bought is really rather 700W one if the Platinum and Gold versions are similarly built and configured. It also has 20AWG wires in PCIe cables past the first connector which makes them near useless. They've saved like a buck at most on that, not the kind of move i expect from a 'top-tier' OEM as some people boast them to do with a 300$ PSU.
Yes, it's a bad choice. Corsair RM-x and EVGA G6 would be better.
Irrelevant, all 'Japanese' capacitors you see in consumer PSUs are of Chinese origin now anyway, made on the same factories as some other 'non-Japanese' brands. Besides, capacitors are not the only important, and not even the most important components of a PSU.
Near irrelevant too, Gigabyte P-GM has 10y warranty too. It's a nice to have for sure, but not a sign of quality.
Doesn't mean anything either. Considering it's a 150$ 700W PSU when you can get some 850W for 40$ cheaper which are better at least because they're well reviewed.
That's kinda the 'problem' with Corsair PSUs but they're stiff because they're 16AWG, whereas even Seasonic uses 18AWG and well, Super Flower is 20-22AWG. There are also in-cable capacitors which make things even worse. But all in all while these cables are complete PITA to work with, if you aren't going to rewire it monthly then it's kinda irrelevant, do it once and forget. There's also Corsair RM and White versions of RM-x which don't have in-cable capacitors.
Sure, EVGA G6 then. It's 15$ cheaper than even RM so it's kinda default choice anyway.