Guys, that was the 4.2 watt SoC in the current Switch, not the system, which is 7 watts at full tilt (brightness full, full volume, bluetooth, wifi, ect)
4.2 watts + 2.8 gives us 7 watt power draw of the Switch when full tilt, so lets assume the new display draws .8 watts less, for a 2 watt system outside of SoC
X1 SoC has a 20nm design with 4 A57 cores that use ~1.83 watts, GPU is just under 2.4 watts. SoC components we care about here are these 2, so
4.2 watts. (this is the target we should look for, give or take half a watt at 16nm)
(I'm going to use Foxconn leaked CPU clocks as well for this, to see how realistic it is)
16nm GPU 256 cuda cores ~1.4 watts.
16nm GPU with 512 cuda cores ~2.8 watts
16nm A57 quad core at 1GHz ~1.2 watts.
16nm A72 quad core at 1.78ghz ~2 watts.
16nm A57 quad core at 1.78GHz ~3.6 watts
21.5wh / 4.6w (2 watt + 2.6 watt SoC) = 4.7 hours [A57 CPU 1GHz + 196GFLOPs]
21.5wh / 5.4w (2 watt + 3.4 watt SoC) = 4 hours [A72 CPU 1.78GHz + 196GFLOPs]
21.5wh / 6w (2 watt + 4 watt SoC) = 3.6 hours [A57 CPU 1GHz + 393GFLOPs]
21.5wh / 6.8w (2 watt + 4.8 watt SoC) = 3.2 hours A72 CPU 1.78GHz + 393GFLOPs
21.5wh / 8.4w (2 watt + 6.4 watt SoC) = 2.5 hours A57 CPU 1.78GHz + 393GFLOPs
The source for CPU power draws below. GPU is more tricky, however the reduction is estimated anyways, these should be close enough for this type of speculation.