How crucial is it to ditribute your exp correctly in the Nohr path?

[Ayra]

Exp punishment while being way too overleveled is pretty heavy this time and vice versa.

Based on my Hoshido Hard playthrough, that's correct. It's to the point that a level 5 class-changed character will get ZERO experience points for beating an unpromoted level 20 enemy. On the flipside, you get a ton of experience if you are even just a little bit behind so it's very easy to catch up, especially when you know that your double-attack odds are 100%. I think it's a good solution to stop strategies like just picking two units and mowing the entire enemy team with them.