Yoshida already mentioned they have no plan to nerf any of the raids. Patch 5.5 will add a new "open area" zona and rank increase. Said rank increase will act as an indirect nerf.
I'd be more inclined to think of an increase in lost action capacity as an increase in character strength if it seemed like more people were willing to use lost actions in the first place
mettle rank is there to gate story progress and the higher level lost actions you can appraise. that's pretty much it. there is no difference battle wise between rank 1 and 15.