The watt numbers I reported were measured externally with a Kill a Watt, so measured the total system power use of the iMac. So 175 watts while the GPU is being pushed hard includes the GPU, screen, some CPU, and other components. At least 35-40 watts are being used by those other parts, so the upper limit of the GPU would be 135-140. That assumes that the GPU was really pushed to the max, and considering the temperatures and fan speed, I think it was.
Aside from that, I'm pretty sure that the linked article just had their facts wrong. 250 watts is the rating for the desktop equivalent. It would make no sense that the mobile chip would have the same rating. I can't put my hands on the sources at the moment, but I've seen reports that puts the m295x at 125 watts. In the end it doesn't really matter. The system is limited by how much cooling can be done. It's pretty clear that the upper limit for heat output for the GPU is about 105 C., and the fan and possible throttling don't let it go higher. AMD obviously thinks thats not too high.
Apple indicates the M295X riMac can pull up to 288 watts, and that's "CPU Max" - not sure if they're including the GPU and display, as well.
http://support.apple.com/en-us/HT201918