Does the highest subcarrier (N) move faster (arrive first) than the first subcarrier (1)?

I suppose you are speaking about a regular fiber, then refractive index is decreasing with wavelength and that is all you need to know. Your modulation creates a broadening of the original laser line on the both sides, at high and low wavelengths, and each of these spectral components will propagate at a different speed according to the dispersion curve of the waveguide (index vs optical wavelength is what matters).