only 20W for 2GHz quad core A57 + 1Ghz 512 FP32 GFlops GPU on 20nm is very very impressive in my book. Saying that it consumes 20W without taking into account the very high sustained performance, is hiding half of the story. Because X1 efficiency is at the top of the market and it can be adapted to a tablet easily.
Totally except not necessarily. You reckon the Tegra is as beastly when sat next to the, say, i7 5600u? That's a 15W part.