The PCIe slot can provide 75W and a 8-Pin PCIe cable can provide upto 150W, I think these are only assumed values & a good quality power supply can provide higher power than these values as long as there is sufficient headroom.
According to the review of R9 295x2 on tomshardware(http://www.tomshardware.com/reviews/radeon-r9-295x2-review-benchmark-performance,3799-16.html) , the average peak power consumption of R9 295x2 hovers around 450W. At load it only draws 28W from board itself which is far away from assumed value of 75W. So this basically means that the 2 × 8-Pin PCIe cable are providing 400+W of power to the GPU.(http://www.tomshardware.com/reviews/radeon-r9-295x2-review-benchmark-performance,3799-2.html).
This is why only...