Very true although we don't know if the game is using 100% GPU power docked either. I would assume Nintendo would use whatever mode that they used for MK8 and that's 1080p60 and 720p60 and obviously they are different game but it's an interesting comparison.
True.
Anyway, I somehow doubt they were utlizing the higher speed RAM of the Docked mode fully as that would have introduced more restrictions than seen here.
However, the possibility remains that while they might not have really maxed out the memory bandwidth in Docked, they still could have used more bandwidth than available in Portable (like ~11% more, for instance, instead of the full 20% more). That and perhaps not enough optimization could very well be the roots of the issue.