I've read that the 6pin provides 75w while 8pin provides 150w, and I believe the black wires are always ground wires, so I'm quite bewildered about how an extra 2 ground wires doubles the capacity

In theory, three 12v wires would perfectly balance three ground wires? (Sorry I'm still in gr 10 science not too good with electricity

).
I've also seen some manufacturers distribute these crazy adapters that split up one single 12v wire into 3 wires of the same guage, for example the one that came with my 4870 that splits up one 12v wire from molex to feed a 6pin pcie, is this stuff safe? Even the HX650 splits a 6pin pcie into 2x6pins, would that mean each plug only receives half of 75w? Or are these just random numbers that the ATX guys pulled out of their ear to discourage us from doing crazy stuff with their plugs?
Thanks