There is a Plus6 Peter.........it's known as the Roadster!
Roadster = Readitional chassis, hair shirt, etc.
Plus 6 = Aero Chassis, Automatic, etc.
Aero 6 = Economy Aero 8 for markets where local taxes make the Aero 8 impossible.
The Aero and Trad bases both have strengths and weaknesses. The result would be simmilar, but very diferent cars. Both could be offered by MMC and I think they would attract different customers.
Actually an Aero 4 with the full ford Ecoboost motor putting out about 250 bhp with the Turbo, would be sensible in places like Ireland, Italy, Belgium and others where large (over 2000cc) engines are stupidly taxed on an annual basis.