A typical 3000 mAH cell phone battery would last only 18 minutes of time just for 10 watts CPU alone, adding Display, wifi, DDR, etc would be less.
For automobile connected or wall powered devices (robot), 10 watts for CPU/GPU power is fine.
In contrast, the TX2 in "MaxQ" mode (energy efficiency savings mode) achieves close to equivalent performance on many benchmarks to the TX1 at half the power budget, so it has 2x the energy efficiency overall.
The real question is where Xavier lands on the power/efficiency curve here, but I'm betting it would be pretty good, and there's nothing to necessarily disqualify a downclocked part, here. I think a custom version of Xavier could make a good gaming part, if it wasn't for the cost being outrageous.