Looking at the latest AMD Mobile processors a 35 watt Carrizo can pack 4 Excavator CPU cores and 8 Compute Units, that puts it in the ballpark of a Wii U in terms of power consumption, but somewhere between the 2-3X Wii U in terms of hardware performance on the GPU side, the CPU is substantially more capable than the 3 core PowerPC, in fact it would probably far outstrip the CPU performance of XB1.
That's a 28nm processor, Nintendo may have access to a 14nm Zen CPU (probably the latest GCN tech for GPU at that processing node), which will feature Simultaneous Multi-Threading, with new architecture overall and a more efficient node that 14nm tech could reach PS4 levels of performance at 35 watts, it just depends on what AMD has available when Nintendo wants to begin production.
I think AMD were hotly rumored to be producing a SOC for one console manufacturer, which would be introduced (which could mean revealed to the public or released to market or both) next year.
Nintendo seems the most likely company to be AMD's partner and this tech is what AMD has available.
Carrizo and a 28nm node would be the most mature, more efficient than the tech in PS4 and XB1, giving Nintendo choice and probably a cheaper cost, but Zen and the next line of tech that comes along with would offer a substantially bigger leap in performance, but costs would be higher, although going with a newer line of technology does let Nintendo futureproof themselves a bit.
Both options could run with a newly designed, unified API and OS, future AMD tech would allow backwards compatibility with previous generations when and if Nintendo wants to introduce the next evolution of their NX platform.
TBH either Zen of Excavator would be a good option for it's own reasons, it all depends on Nintendo's design options.
The GPU tech in Carrizo is updated, with a richer feature set, the CPU tech substantially better per core compared to PS4 and XB1, 4 cores may outstrip XB1, the GPU would be a fair bit weaker though with 8 compute units, Nintendo could always use a design packing the tech of 2 Carrizo APUs onto one die, 8 Excavator Cores (remember Excavator cores are much more capable than Jaguar) and 1.6TFlops would probably equal or exceed PS4 in performance, at 70 watts power consumption for the SOC, that's an efficiency gain of more than 30%.
It all depends on what Nintendo wants NX to be.
