Mirror aerodynamic development is mostly dictated by the aim of reducing the drag of a flat reflective surface. Curving the top and bottom surface around substantially reduced drag. However, any 'boat tailing' added to a baseline mirror configuration would still generate a wake behind that sends lossy flow on top of the sidepod and towards the top of the diffuser.
Ferrari opened up the front of the mirror, which fills in the wake behind the mirror with a fresh, more energetic freestream air. It also decreases profile drag as the pressure difference between upstream and downstream of the mirror is reduced because of this channel (top and bottom duct).