Yeah , wasn't also the problem for Nvidia the fact that they'd have to pay big penalties if they didn't order at least some more millions of units from TSMC on the 20nm process? That could also point toward Nvidia giving Nintendo 20 nm to avoid said penalties. Take this with a grain of salt because it was something i remember reading but could remember wrong or could just be a fake news.
As for the screen, i checked for the most common, cheapest screens around 6-7", and they seems to be 1024 x 600 (614400 pixels), and cost about 1 dollars on ebay, i suppose they'd cost like a tenth of that to a big manifacturer. Compare to 1080p which is 2073600 pixels. This would imply a ratio of 3/10, or a 300 Mhz speed in mobile vs 1 GHz full clock X1 on dock. Not sure how much less does a X1 consume at 300 Mhz, if it's feasible in term of handheld consumptions. At that speed, you would be looking at 153 Gflops, or about WiiU power in HH mode (assuming a stock X1).