Starting to feel like all but Crane formation (SPD) will be worth using by the end. I've seen some enemy units with 10k Atk and players already up to 8~9k+ Atk, I'm sure some are already higher. The +250 Atk from a Lv25 Python formation or even +500 from a Lv50 Python (assuming it is +10 every level) is not gonna be much of a factor when you have 10k+ Atk, while SPD has less ways to be increase, being more valuable in going first in battle and some units such as Tiger Kai need higher speed just to do their big hitting skill.

Or does anyone know if formations get a new bonus after a certain level? The JP wiki lacks a lot of minor details.