Is elastomeric better with more polymer, or with added calcium carbonate?
Stuff like calcium carbonate is generally used as a cost cutter, as it is far less expensive than even the cheapest elastomer (polymer). It allows the producer of the formulation to make a profit/make more profit, essentially.

Generally speaking, adding calcium carbonate to an elastomeric formulation will decrease the useful properties of said formulation, including its resistance to weathering. It's resistance to wear and tear (as represented by mechanical properties like tensile strength and abrasion resistance) will also suffer. Calcium carbonate containing formulations, all other things being equal, will also be less elastic and somewhat stiffer.

A polymer that has poor resistance to weathering (like SBR rubber) will generally also not benefit from calcium carbonate as a formulation ingredient. Instead, special anti-oxydant/anti-ozonants will improve this cheap polymer's fight against the elements.

But if a composition does contain significant amounts of calcium carbonate it is not automatically disqualified as suitable: that will largely depend on the end-user's expectations. In other words: cheap can work well too.