If all you are doing is basic insert update delete of things like users and basic objects, then your performance hit will be marginal with a web API.

The more interesting question/application would be if you were manipulating LOTS of data (1000s - 100000s) of rows per transaction. Then you would have a use case that was more suited to defining a lower level communications layer...but what would that application be that was running on a phone?

General rule of thumb is to not over engineer your application.