akv,
Something that many miss regarding color photographic film, whether negative or positive, is that they too have a Red, Green, and Blue layered architecture.
The Foveon imager architecture has no impact on the sensitivity. Red, the deepest light sensing junction, is just 2um below the surface. The design is based on the quantum interaction of photons and the atoms in the Silicon crystal. Short wavelength photons, in the blue region of the spectrum, have their highest probable interaction just 0.2um into the crystal, where as the long wavelength photons, in the Red region, don't interact until they travel ten times further.
The stacked architecture does not reduce sensitivity across the spectrum and in fact Reds end up being easily saturated even though they are sensed at the deepest level.
I've done my home work and prior to my SD14 purchase, I searched out and read the patents that were issued to Foveon for their technology. If you're up for a techi read, here's one of them: