panelling under axle/ rear diffuser

[Not Anumber]

Ive read claims elsewhere that fitting a rear diffuser reduces drag and noticeably increases top speed and ecconomy by cleaning the airflow under the car.
As far as I can make out these diffusers are a sheet of metal or fibreglass with fins.
Does anyone have any views on whether this is the case and if so could the same be achieved by fitting a flat ally panel under the rear axle and tank so that the floor is effectively extended right to the back panel ?