It won’t make that much of a difference in performance if you’re not using a lot of masked stuff, with a bunch of overdraw, like trees and grass. You can see if it’s working by looking at a masked actor in material complexity view mode (Alt+8). When “Mask material only in early z-pass” is enabled, you should only see the masked part of the material. If it’s not working, you’ll see the masked material the same as you would a transparent one: the transparent parts cause overdraw and are colored more towards red in material complexity view mode.