Hi Marcco, the 8-10deg diffuser angle quoted was applicable to the flat bottom formula cars of the early 90's and it changed (the optimal angle got bigger) with the change in the rules specifying stepped floors for the F1 cars (that's pretty much what I do for living). As a general rule of thumb, the higher the distance between the floor of the car and the road, the higher the optimal diffuser angle. Also if we accept the theory that the diffuser will add equal amounts of downforce to both axles of the car (which probably wouldn't be the case with a bit more going to the rear being more probable), then higher diffuser angle than the one on the Fiesta would look more appropriate.
Regarding the double diffuser, the only way it won't be doable is if the rules do not allow it (for one reason or another), otherwise there is no stopping the creativity of the designers.
Regards,
Nick