They are different chips but the performance is close so I don't think that will change with using X2 instead X1.
X2 indeed has a slight better perf/watt and a better memory bandwidth.
Is it enough to make difference in graphics? No.
Maybe Nintendo wants to use Tegra Xavier but that chip really exists?
www.anandtech.com
Xavier indeed will at minium double the GPU/CPU power over X1 (Erista ) / X2 (Parker).
Edit - Seems like there is one device using Tegra Xavier:
www.anandtech.com
But the cooler is not something you will use in a portable and this chip was not developed to the gaming market.