You can probably tell, but I've never been a big fan of Quantities because of what I see as large overhead. That said, I understand that the majority opinion is against me on this, and I'll accept your PR onto my branch. 

However, can you include making the cython functions public (and documented) as part of that PR? I think that will be a nice compromise. In fact, for use cases that really need performance, this will be even better because this will skip the wavelength check. (Leave it to the user to do this when using the low-level functions.)

The only downside is that we can't do this for F99 and other spline-based models (they are not self-contained in Cython code).