Dx11 only improves performance if the developers optimizes for it. Performance may actually drop if they just did a simple switch over.
From the way Yoshi gives his interviews, I assume they are actually working on the optimizations.

In which case...
I am hoping for a environment boost with Dx11 Tessellation - No more flat grounds, cliff walls...etc
- With less polygons needed to maintain the same graphical fidelity, they have more options...E.g. Even more characters on screen at the same time, bigger zones perhaps?

And better & higher performing lighting effects. E.g. Ambient Occlusion.