Yes, as long as the brushes use the same texture flags etc. then it should optimize it this way.
Don't forget to split the mesh is clusters after the union, this will give you a high speed based on a good relation between surface count and Tris rendered. Clusterisation will double some surfaces, but don't worry too much about Single surface. The card can easly handle some dozens of sufaces, if the number of triangles is not too high. You should not reduce the surface count at ALL costs. Because, what ever is part of a mesh that lies within the camera viewrange, it will be rendered, even if most of it is behind the camera, or out of its range. That's why clusterisation makes sense.
Don't forget to split the mesh is clusters after the union, this will give you a high speed based on a good relation between surface count and Tris rendered. Clusterisation will double some surfaces, but don't worry too much about Single surface. The card can easly handle some dozens of sufaces, if the number of triangles is not too high. You should not reduce the surface count at ALL costs. Because, what ever is part of a mesh that lies within the camera viewrange, it will be rendered, even if most of it is behind the camera, or out of its range. That's why clusterisation makes sense.