Nintendo already confirmed there is no additional hardware in the dock, just HDMI and power. And I highly doubt Nvidia is going to manage to get 400+GFLOPS from a custom chip that's going to have to be portable as well, otherwise they'd have to severely underclock the thing just to keep battery life somewhat "ok" which will in turn kill performance in just about any game.
Face it, the Switch isn't going to be a power house period. This won't be much of a problem for Nintendo, since they can squeeze every ounce of power with their own games that look good, but third party devs are going to have to downgrade their games if they want them to run well at all docked and "ok" when it's portable.