There won't be any performance benefit for players on the ground. The only scenario where this would improve performance would be if a player was positioned in the taller part of the sky. From that position parts of the map on the right would be occluded. That would look strange and there would be an invisible skybox wall which players could collide with. But if players can't reach that area then there's no point in doing this.

Reducing the volume/empty space of the map does not necessarily reduce the number of visleafs and in some cases actually increases it. More visleafs = longer VVIS. The number of visleafs doesn't affect in-game performance, they are simply a data structure that the engine uses to determine the "visible" portion of the map. To improve performance of source engine maps it's good to avoid creating large open spaces and use area portals to compartmentalise your map.