Let us say that a caster is 20th level.
Casts Ephemeral I and gets +4 to temp, limit of 2 over potential.
Casts Ephemeral II and gets +8 to temp, limit of 2 over potential.
Casts Ephemeral III and gets +12 to temp, limit of 2 over potential.
vs
Casts Ephemeral I and gets +4 to temp, limit of 2 over potential.
Casts Ephemeral II and gets +8 to temp, limit of 4 over potential.
Casts Ephemeral III and gets +12 to temp, limit of 6 over potential.
A 20th level caster giving themselves a 102 in something (or perhaps, at a stretch a 103 or 104) is not the same as getting a 106 or pushing it 108. . .the problem lies in the way the stat table works, as every point over 100 is worth more bonus. . .so a 108 is way off scale. . .
2 points can be worth +10, 6 points can be worth +30. . .
The first set is already pretty potent, and is the words of the spell as written, the second is a stretch, assuming the potential gain scales as the temp gain. . . .
Would you assume that a fire wall II spell that was "As Fire wall, but a B critical" implied automatically doubling the AOE, duration or Range also, or just that the spell is exactly identical, except it does a more powerful crit?
Going purely by the RAW, the up-scaling is to the temp gain only.