I'll say below.

What makes you think they wouldn't use the full power? They have done so on the Wii U. In fact, what they have managed to do on the Wii U so far is quite impressive.

That hasn't been said for a long time actually. There was an old quote from Iwata used about the Wii U when it was released, that has resurfaced in speculation about the NX. As far as I know, the only thing I have seen about NX power is that dude on twitter. Although, with that, there was a crap ton of rumors about all the consoles before they were released, some saying that they were way under powered and some that said they were ultra powered. In the end the only ones that knew were the designers of the machines because the dev kits didn't have specifications due to them not being finalized.

It really is too early to say mostly because I don't know how expensive the innovative features will be. If it ends up being a standard console and a handheld which share OS i think the console will be slightly below ps4 overall.  Basically it will be at about Xbox One level of power but be more efficient with wiser decisions made since Nintendo has been able to sit back and anaylize what worked and what didn't work. The handheld I would expect to be slightly more powerful than vita.

Why would Nintendo ever not use the whole power?  Sure for some games they likely wouldn't but with every nintendo machine they've always made at least some games which use the whole power.

I would take what the unseen 64 guy says about Nintendo with a grain of salt after being wrong about the Devil's Third. It's also possible he has info on the nx handheld but not the nx console.

A little weaker or about equal. If it's a home console, of course. If it's a hybrid, portable, or whatever... then that's a whole different story.

Below in power...

going on a console race, 3 years late, similar price point and 60M, or so, behind the other 2 consoles is just suicide.
Nintendo will fight whit a good hardware ( well below in power, but still good), good lauch games and a aggressive price. perhaps something like $200-250 and a great games like Zelda and pokken.
Anyway, they are not going to top ps4. PS4 is going to score Wii and Original PS like numbers, 90-100 million sales, minimum. NX will make profit, but will not hit amazing numbers, at best it will hit N64 like numbers.

Actualy this is recent statment from Miyamoto:

"What we are always striving to do is to find a way to take novel technology that we can take and offer it to people at a price that everybody can afford. And in addition to that, rather than going after the high-end tech spec race and trying to create the most powerful console, really what we want to do is try to find a console that has the best balance of features with the best interface that anyone can use.

But really what's most important to us is, how do we create a system that is both unique and affordable so that everyone can afford it and everyone can enjoy it."

Far below, it's a handheld.