I did it for Eirika/Ephraim because it encompasses six midgame chapters in a game with ~20 maps total. Well, slightly more due to Prologue, 5x and two Final parts, but the first maps are super short. In this case, we talk about two maps in a game with ~30 maps. 1/3 of the game versus 1/15th. I believe the impact for most characters is negligible in FE6 compared to FE8.
For example, on Ephraim route, Duessel is considered one of the best, if not the best character when he joins. On Eirika route, he misses like 4-5 midgame maps, dropping him several tiers.
Per comparison, the non-route-exclusive characters Gonzales and Geese hardly differ much at all. I made a huge point of this on SF: if a unit is better than Geese [A], then he is better than Geese [B], no reasonable way around it.