All true, but there’s a reason for lamp shades as well, as they diffuse direct light and make the illumination of a room friendlier. Glare is a real issue and so while placing a mirror next to lamp increases the light in a certain direction it could be uncomfortable looking into the direction of the lamp. A white wall would reflect the light in a more diffuse and thus agreeable fashion, but the overall output measured in the room is gonna be smaller. Illumination depends on your needs in the end.