The 75W limit of the 6-pin connector is laughably under specified. Look at the datasheets of the components the mfg would use and the sustainable amperage is almost certainly much higher, especially if they use quality plating. In your test the draw from the power connector doesn't seem to go past the 78W which makes me believe the 75W is a "hard" limit set inside the PSU when only 6-pins is detected. Using a 6+2 and shorting the unused +2 works around that, the PSU should be able to supply 150W, more for the conn. to use, see how the load distributes then?
If I'm right, anyone with an RX480 and a 6+2 pin capable PSU can 'fix' their card without any power concern for the cost of a paperclip and about 5 minutes of their time.
I don't have an RX480, I wish I did, so I can't try this myself. I want to know what happens if you plug a 6+2 style connector into the card instead, and short the +2 pins to force the PSU into 'enhanced power mode', the same thing a real 8-pin card would do. Also, I'd like to know if the middle power pin of the 6-pin connector is actually connected to the other power pins or is it just there for cosmetics. Not a huge problem if it isn't but it would be better if it was.
Shorting the "+2" would drive the sense pin that is required to unlock the additional power (for "8-pin" cards) and should offer additional card power.
I build motherboards for a living, and their statement about "Standards exist for a reason" is right. You should follow them or else the standard is meaningless but breaking it isn't illegal (unless you say something is compliant when it isn't) and almost any device worth buying from a reputable vendor is going to exceed it the specs. The build quality would have to be exceptionally poor and if it were you would most likely have further problems.