Cut-out CAN be more efficient than translucency, for two reasons:
- The graphics card may not have to read what’s below the pixel to calculate a blended value
- The graphics card may be able to update the Z buffer in a way that makes early-Z testing work
The specifics of these two options depend a lot on the specific hardware you’re running on, AND on the scene.
For example, if nothing will generally be behind the clouds, the the early Z testing doesn’t matter, because there’s nothing to occlude.
The only way to know whether this matter for your hardware and your art is to set up realistic scenes and measure on realistic hardware.
If the clouds aren’t large, covering the screen with translucency, but instead the edges are smallish fringes on the screen, then the translucency will probably not be a problem.
Large transparent areas, especially with a lot of overdraw, will be a problem, especially on mid-to-low-end hardware, though.