So, MK8 is now at +5m shipment (5,110,000), and 340,000 this quarter.

The legs of this games are insane... so, do you think will sell more tham Double Dash, and why no, even more than MK 64?

Eventually over MK64 assuming 2 factors.
1. NX is actually a handheld that is powerful enough to run Wii U games.
2. If not that, then MK8 gets more DLC as Nintendo said it would in its financial report. Nintendo then proceeds to bundle MK8 with every Wii U.

MK8 AR is around 55%. Assuming what can be the Wii U sales:

15,000,000: 8.25m (above MK DD)
16,000,000: 8.8m
17,000,000: 9.35m
18,000,000: 9.9m (Above MK 64)
19,000,000: 10.45m
20,000,000: 11m

Maybe less, maybe more, of course...

A handheld powerful enough to play WiiU titles...

Right?! It would be insane! Plus I think its feasible.

OP?

I hope it at least outsells Double Dash! MK8 doesn't deserve to be the worst selling console MK game.

To run Wii U games, it would need the same CPU. You can't put a PowerPC CPU on a mobile device. It won't have the same GPU power too. The most powerful SoC available is the Tegra X1, that is similar to PS360. This SoC can't be used in a device with similar size to a smartphone, you need a tablet-sized device for it. It also is expensive and it's being used on 300 dollar devices. None of this information helps to fit it on a portable, at least not in 2016.

To run Wii U games it has to:

- A: use the same hardware. Inneficient for a mobile platform and most likely technically impossible in 2016. Despite that, Wii U's technical project is flawed, it's not interesting to carry it for a new device.

- B: use other hardware and emulate it. Impossible due to power constraints, since you have to exceed by far the power of a platform to be able to emulate it;

That's why they (people who predict )trying to say "Fusion " (Iwata said : "it will redefine console and handled ) i think it's possible, if the next platform will be as much as powerful as console but with portability form factor. But i guess it will not as powerful as PS4 or Xbox One if they trying to lunch on 2016 - 2017, so as powerful as Wii U is quite possible.

Edit: at OP I think it will be under Double dash , Wii U just simply dont have a lot of userbase, it's different story if they can achive sales on par with Gamecube. But I believe it out sell Mario kart N64, because Mario Kart is now more popular then when it still on N64.

Edit twice