This has actually been done many years ago. http://django-nonrel.org . Don't know how it differs from this one, but it has many of the same features. Used it for a project 4 years ago.
How is performance compared to using cloud sql ?
How is performance compared to using cloud sql ?
As for performance, it depends what you are doing. Obviously the datastore is non-relational so it scales much better with a lot of data, and you don't have to deal with things like migrations or connection limits. But then you can't do complex queries and aggregates etc. the good thing about using Djangae is you can make use of Django's multiple database connections to store your data in whichever form makes sense (datastore or CloudSQL).