Try it out in a field. My guess is you'll need a much bigger island.
According to my throw simulator, for a circular island with the target in the center and the front lip 275 feet from the tee, the island needs to be 180 feet in diameter for 2/3 of advanced players to land on the island with their first throw. (This places the target at 365 feet from the tee, 90 feet in from the front of the island.) That's about 25,000 square feet, where your proposed island is only about 5,000.
Now, if the distance to the target is 275 feet, you can get by with a 140 foot diameter - or 15,000 square foot – island. (The front lip will be 205 feet from the tee.)