The one thing that seems to have been overlooked and discussed, is the purpose of the core. From my point of view, it just opens up the gap between the topsheet and the base. This is where the real strength of your ski is created . If you want a 'real' stiff ski, build a pair of skis on a thicker core.
I work in the aerospace industry and have a little to say about this 'foam' that everyone seems to be unsure of. We use foam core in varies applications in the design and manufacturing of composite piston aircraft and personal jets. On our shop floor there are RACKS full of foam. They come in all different thicknesses and densities. There are some sheets that look like sponges (large air voids) and some so dense that they seen solid. Some have commented on the quality of these cores and that is subjective just like the wood used for the wood skis (as noted about softwood and knots). From my experience this stuff is just like working with wood. We work with it in the same way that you do wood. Cutting, milling, chamfering and drilling it is all the same. I would bet that most of the foam that is made for the composite industry for the most part is coming out of the same plants. (Higher quality standards for areo of coarse)
If you discuss foam in the terms of a injectable goo, them yes a foam core ski is junk (no control). If you talk about building a ski around a milled and 'consistent' core of foam, them I think that it would be on par with any wood cored ski. I would go so far as to say in the ski building biz, you could mill foam and wood all in the same day on the same machinery.
At the end of the day I wouldn't use the core material as my starting point for purchasing a new pair of ski with the technologies that are in place today. There are much larger parameters in my purchase decision (size, cut etc).