Forgive me if I am missing something, but wouldn't moving the cookies to localstorage still require an additional call to be made to send the data to the server in the end - not to mention the additional javascript required to handle the localstorage management and ajax calls?
The idea is just to get rid of the extra overhead on the HTTP request, because that overhead will be multiplied by the requests you do. Any extra code on ga.js is cached and doesn't incur in too much overhead.
But you would still have an additional HTTP request that would be sending the same information + headers for another call - in the end it would be the same performance wise as if you were DEFERing the request, but you wouldn't be saving any overhead on data passing around.
