I use the phantom for racing and wouldn't use anything but camed sails. If you are not racing, then I can't see why you wouldn't use what ever you feel like using or alread own. All the sails are a balance of power, weight, stability, price,etc. I have the phantom 8.5 sail which Remi recomended and is a very nice balance of power and weight and works well in strong and light wind. I would also recomend it.
You shouldn't buy it because the board deserves it. you should buy it if the sail you have is not meeting some need that you have.