Don't care. Looks good. Want more "looks good" games.

Of course it's not hitting its limit; it's too early in the system's life.

He even says in his comment: "I think it still has a lot of potential left, and I suspect there are more ways to make use of its capabilities that we haven’t even imagined."

By 2016 we'll have seen Wii U games that are a clear step above MK8 graphically.

It's easy to push a system to it's complete and utter limit.
You can't make the hardware "do more" than it's actually capable of.

However, on the software side, there is LOTS of wiggle room, the WiiU is a new system with different strengths and weakness's compared to other machines and it will take awhile for game engines to match those nuances.

The wiggle room is what I'm talking about when I refer to its limit; not just throwing raw code at a machine until it chokes, but fully optimizing and utilising all its components to get the best possible results.

I expect future games like Art of Balance, F.A.S.T. Racing Neo, and Monolith's Soft's X (as well as other games we don't know about yet) to exceed the bar being set by MK8.

Not only is it a racing game but its also a kart racing game, I can guarantee that its not pushing the systems to its limits. GTAV or TLOU on last gen is more like pushing the limits.
Plus the WIi U has the power of the cloud.

That's like 3.142791 Xbox 325s! Lol. I agree though, Retro is supposedly tasked with making a new game engine to vastly better utilize the architecture of the Wii U. Not just try to force Cryengine 3 to work on it.

Mario Kart 8 size is just 4.9 GB. If you think this game push Wii U to the limit, imagine what kind of graphic will be make from a dual layer Wii U game.

No need to point out that later games are going to push it more, he even says there's potential left. For now, it does push the Wii U technically. If you have played MK8 already in 4 player split screen, you know what's going on and compared to other games on Wii U it really is technically amazing.

This x10000