Well in comes RUF to do what everyone wants by putting the 3.8 liter motor out of the Carrera S into the Boxster calling it the 3800S. The PDK transmission is retained and the Boxster now has 420 horsepower versus the 315 horsepower Porsche gives it from the factory.
The thing is, one does not need RUF's expensive swap. The 3.4 liter and 3.8 liter motors are basically identical. What is the main difference? The bore. The Boxster S and 991 Carrera have the exact same 3.4 liter at 97x77.5mm. The 991 Carrera S gets a bore increase to 102mm yet the stroke stays the same 77.5mm.
So take a Boxster S, bore it out, get some factory Carrera S pistons, and voilà. A 991 Carrera S motor in a Boxster.