Perhaps 50mm mirrors will become the norm on subwoofer drivers? 25mm Xmax?
The theory is good but the mirror is just too large for normal drivers with conventional dustcaps. A narrow strip of mirror would do but what would it be made of to reflect a narrow beam with any accuracy onto a target? It would require flatness and any flexure would make it impractical. Glass would probably modify the driver's behaviour due to its weight and might be a potential risk if it detached itself. I suppose space blanket foil could be glued to a suitable cone shape. There is a trend towards bowl shaped driver cones with inverted dustcaps so the potential is there to apply a reflective foil directly. Mirror polished aluminium cones are not impossible?