This isn't something that can be balanced easily.
Even accounting for stats being synced, base and gear, your level 80 rotation still deals many times more damage than a level 15 rotation, because you acquire countless skills that increase your damage by a %, or your base potencies, or you gain more oGCDs to weave between your GCDs, and these all stack and compound your damage output.

In order to properly balance the level 50 Extremes for level 80 movesets, they had to basically fine tune each specific fight, and that's what became Unreal Trials.