troek: if that matters that much, one can also think of adding rad to cool diff oil (as i've seen for eg. GT86 CS V3 Cup track cars). Still imho nothing that would prevent making these by vendors. If even having or not stock underside panels slightly affect mpg (if that is any of criteria to judge aero efficiency
), then imho full/even flatter one should do even better, easing high speed for underpowered NA.