I will say 1/3-1/2 of an XBox One in actual real world performance.
Memory bandwidth is going to be the big problem for the Switch since I don't think they can realistically use high bandwidth eDRAM or eSRAM in a portable.
And even going with a 128-bit bus for the RAM is a problem potentially because that would increase battery usage and cost. My guess is they're making due with 25GB/sec 64-bit bus but leaning heavily on Nvidia's tiling approach to reduce bandwidth as much as possible with some faster memory caches.
If this thing is really capable of running PS4/XB1 ports, why in the world would you show a five year old Skyrim primarily in the teaser trailer? That Mario game also looked nowhere near a full generation leap past Mario 3D World, it looked very marginally better. Also Eurogamer's sources, which got everything basically right about the NS say 2-3x better performance than a Playstation 3 (not 4).
Which for a portable system is pretty damn good. If you were expecting more than that, I just don't think it's reasonably possible with today's technology.
You can't have a 20 watt portable, your battery will be dead in an hour.
