At long last, Treasure Isle on Facebook introduces Treasure Mastery -- which can be viewed in the newly revamped Treasures collection in the game. Now, when you complete a collection, you'll be able to trade it in for a reward, which generally includes a bunch of XP points and gold or a rare item.
To view your Mastery for a collection, look at the row of four stars at the top of the collection (as seen in the pic below). Right now, it doesn't look like the number of stars will have any impact on your reward -- you will get the same bonus no matter what level you are. Double checking on this new, and will report back.