Waiting for experts to chime in, but I do believe that longer runners are good for torque, while shorter runners aid in high RPM HP. That is why most systems out there, varioram included, will effectively cut the length of the runners as the RPM's go up. I think that the main reason for using the Carrera intake is the simplicity of running the fuel rails (you can just buy existing, proven 3.2 rails and lines), coupled with the better flow characteristics and port sizes, and also, because it has become the norm. It's kinda like the engine equivalent of a ducktail spoiler for an early car! Plus, the basic runner/plenum design is very similar to later manifolds, like the 964/993 cars.
The design of the 3.3 manifold seems to have been compromised more for packaging issues than for sheer flow. It was the only way they could get the CIS hardwar, the IC, A/C, smog pump, etc under the lid. They probably figured that the positive manifold pressure would compensate for the intake's shortcommings.
An interesting test would be to run a non-turbo motor with an EFI/3.3 manifold, vs the 3.2 Carrera. You would really see the differences then.
On another note, it is intersting that the TPC 993 supercharger package uses the basic 3.3 pancake manifold design as well. Packagin needs, I suppose.
Garen (87 930 track rat)