They already have 360 parity. Tri-core PPC processors (Wii U is clocked to half of 360 speed but CPU was never really the problem with those games) with AMD GPUs bound directly to the southbridge (Xbox 360 has an R500 equivalent, Wii U has a Radeon HD 4000/R700 equivalent). Also, the Xbox 360 had 512MB of VRAM and no other RAM; Wii U uses 2GB DDR3 with 32MB transparent rendering cache.
The Wii U does more than the 360 with less, already. It's actually a pretty great piece of engineering. And there are already tablets that can kick butt in that form factor (https://docs.sony.com/release//specs/SVF15N17CXB_mksp.pdf) so it's not unreasonable to think that with the new AMD Polaris tech that they're going to make something awesome.
Having learned from the Wii U, I think Nintendo is going to knock this next one out of the park. My biggest questions are, will there be 4K support, and what's the battery life on the tablet mode?
If you looked at that PDF, you'd see a battery powered tablet that has an Nvidia card matching the Wii U. And it was released some time ago, before Polaris.
As a user of AMD's current Polaris tech, I'm certain this next console will kick butt.
If you think of the use cases they showed in the demo, they're not using the battery for extended amounts of time. The guy was playing while his dog did its business. That's a few minutes, maybe 15 minutes. Maybe half an hour if the dog wants to play a bit. My dogs usually tire themselves out at the dog park in about that time.
The guy playing at the airport, you might be waiting up to an hour. When he got on the plane, he plugged it in.
The guys at the basketball game seemed like they were killing time while they cooled down, maybe up to an hour.
The person at the rooftop party, she was at a party. They're probably not playing for hours.
The people in the car, I don't remember if they plugged it in or not but they could have, or they'd be playing on a short trip.
The one case where the person is in tablet-mode for an extended period of time, he sat down and immediately plugged it in. I wouldn't be surprised if we find an hour or two of battery life with the most taxing games.
My guess would be somewhere around PS3/360 performance, perhaps with better graphics.