The effect of Warp Core Potential is the same beneath and above 50 but only up to 70. Its +5 (fully specced). Its the effect of Effiency that decreases, which is intended.
While the Tool Tip and the Dev Blog say, that Warp Core Potential boosts all energy levels, it only affects the settings beneath 75. For example my setting moves from 77/70 to 75/75.
So if the setting is above 70 neither Effiency nor Potential has any effect on the energy level.