Wait.., you can do this easily by using a pre-made rounded corner mask. You only need a single mask object, along with the sprites that you want to have its corner rounded.
Steps:
1) After creating each sprite, spawn the mask below it (1 z-index below it)
2) Pin the mask to the sprite
3) Set the mask's blend mode to "Destination Out"
3) Set the sprite's blend mode to "Destination Over"
Also, keep in mind that enabling "Force own texture" can slightly reduce performance on mid-range devices but this method does NOT require it.