Material is not an issue. It has to do with the overall design of the cone structure, as to what determines rigidity. You can design a paper or poly system to be ultra-strong or have an aluminum cone that is too weak. There are plenty of cases where aluminum cones are bent/deformed due to insufficient strength/rigidity. And in any event, do you realize aluminum, in cycled stress, must always have a finite life, then it will suddenly fail (eventually)? You can go either way..... weak aluminum/strong aluminum ... weak paper-poly/strong paper-poly.
And as for the linked GR Research woofer. I have no data on this product. But GR historically has provided high value products legitimately - so it is probably a good product.
Also, mass has NOTHING to do with bandwidth, even though they imply it in the first part. Mass has to do with sensitivity. Plain and simple. I also fail to see how the lower mass can have ANY effect on low level dynamics.
-Chris