Soundwave on 01 August 2016
For a portable, yeah Tegra X1 performance would be great. When coded to the metal and specifically tailored games for it, you could probably get even PS4/XB1 ports at 960x540.
I think they will opt for the X2 though because they could axe it down to give similar performance, but likely at a much lower power consumption. Tegra at 20 watts max load is way too hot for a portable, but if X2 can bring that down to 8-10 watts, now you're in business.
