The 75 watts is not supplied by the powered riser. The main power for the GPU comes from the six or eight pin 12 volt power connector specifically designed for high end graphics cards. The power to the riser replaces the supplementary power that would be provided by the PCI-E slot which is not much power at all.
really? it was a scandal that polaris takes more from pcie that the spec allows
http://www.tomshardware.com/reviews/amd-radeon-rx-480-polaris-10,4616-9.htmlEDIT: "We skipped long-term overclocking and overvolting tests, since the Radeon
RX 480s power consumption through the PCIe slot jumped to an average of 100W, peaking at 200W. We just didnt want to do that to our test platform."