My RX580 OC with i7 6700 has no problems beating the xbox x version, its slightly more powerfull then the One X gpu but its ballpark One X GPU. Yes the CPU is about 100x more powerfull but its impossible to match the Jaguar as theres no tablet cpu's avaible to PC as of right now.
Well, ive never seen it with the 8th gen, its just not there. I see too many GTX660, GTX670, 7870 etc being compared and often even performing better. In no way is the PS4 even with optimizations matching a double as powerfull PC.
Your arguments of 2x the power needed holds true for the 6th gen, where API's and Windows were quite pre-historic. OG Xbox wasnt more then a PC, P3 733, Geforce 3 with extre vertex shader, 64mb ddr pool and a harddisk. It ran Doom III very respectable, even with DD5.1.
Even Far Cry, Half Life 2 were running great considering.... These were high-end pc games for their time, but ran on a P3 733/GF3 system with just 64mb ram. Then you needed about 2x the GPU and CPU power, and much more ram to boot.
These days though, after almost two decades of improvements in API's and windows, this isnt so much the case anymore as it once was, and i suspect optimizations to increase in the future. Even in the console space, devs need to code for two or more platforms from just either MS or Sony.