My problem is that I don't feel all of the elements are equally powerful.
As an example, the Elemental Savant (Air) gets much more power from Elemental Perfection (10th level) than an Elemental Savant (Water).
They are less likely to hit their opposing element (acid vs. fire) and almost as likely to hit their favored element (electricity vs. cold). In my experience, it is also much more common to be adventuring someplace where everyone is flying than where everyone is swimming.
It seems to me that resistence and immunity to cold is slightly more common than protections against electricity.
As a Sahuagin or Merfolk, this specialization might work OK. In general, taking that last step as a Water Elemental Savant is a real bummer.
I was looking to play a Water Elemental in a sea faring campaign. Even there, it was very questionable.
There is also the question of what happens if they die? Raise Dead and Resurrection aren't supposed to work on elementals. There are rules that suggest it will work on a "Native Outsider", but none for a mortal that has turned elemental.
It is a very neat idea, though.