I do not think he is over payed. We have to remember that the value for the franchise is a theoretical value based on the price other Nba franchises sold for in the past. It is not realized until the team is sold. Their operating income was 74.5 million in 2016 and by doubling Steph's salary along with contract negotiations with other players it might put a strain on their operating income. However seeing that there revenues have nearly doubled over recent years it might just be the right value.
Plus you have to look at the "projected revenue". The new TV deals are just now starting to kick in, which is why earnings are going way up across the board.