I think it's more likely that the final hardware is TX1 or something near identical.
According to a eurogamer piece on the development of the WiiU, dev kit hardware was similar to final hardware throughout the development process. I think Nintendo fans should brace themselves for the likely scenario of a portable TX1, which given historical precedent is actually a laudable accomplishment for Nintendo.
The Pascal Tegra wouldn't have even existed until recently anyway, thoughh. There's the chance that Nintendo simply uses what's avaliabnle at the time of development and ignores anyuthing newer, but given just how similar Maxwell and Pascal are as well as the fact that 20nm is a disproportionally expensive process due to not being very good and either dropped quickly or skipped completely by nearly everyone, using a custom X1 doesn't seem likely. Also, I think the cost of adding a fan and heatsink is higher than the additional cost of using 16nm.