The Panamera is a success and taking the platform to a coupe design certainly would drop weight. It would also make for a much sportier car. Porsche is not new to GT style cars or front engine V8 coupes. The Porsche 928 is perhaps the most noteworthy example produced from the late 1970's to the mid 1990's with a variety of V8 motors.
Porsche is maxing out their plant capacity as is with the demand for the Macan and Cayenne. A new smaller sedan will only add to the strain. A Panamera Coupe to take on the BMW 6-Series seems logical but it will only happen once the Panamera gets its redesign. The next generation Panamera platform is expected to be shared among several VW brands and serve as modular architecture. When the new car comes this architecture will easily scale to a coupe.
A new 928 sounds like a slam dunk to PorscheBoost. It also sounds inevitable.