A VW four would be more original and provide the ablity to produce a lot of power for such a light car (RCR904 Superlight?).
The Subaru would be able to supply a lot more power than the VW, 300 plus stock Si anyone? Turn up the boost and your quess is as good a as mine but this version strays away from the all Porsche thing. (Your choice, who cares.)
As for a Boxter setup, it would be all Porsche. A six cylinder 904 was built, so it would be valid as a version did exist. (Splitting hairs between water cooled and air cooled.)
Air cooled Porsche 3.2 with G50, HMMMMMMMMMMMM.
Answer to Fran's question, yes the ablity to drop in a six has value.