This idea of "extra biological filtration" is a myth, as I will briefly explain. But first, do not use any so-called "plant" substrate with cories or other substrate-feeding fish. "Plant" substrates in reality provide no tangible benefits for plants to begin with, but some of them can cause serious issues for fish, from bacterial problems to roughness.
I am not aware of any "undergravel" filtration plates that work with sand as the sand clogs the plate making them useless as filters. But there is no need for "extra" biological filtration because it does not exist. The biological filtration is carried out by nitrifying bacteria/archaea and live plants. Plants take up ammonia/ammonium, and fast-growers can assimilate an incredible amount of ammonia/ammonium. Assuming the tank is not biologically overstocked or somehow biologically imbalanced, no filter would be better than adding a filter. In such tanks, filters are primarily there to ensure adequate water movement involving surface disturbance, and providing mechanical filtration (keeping the water "clear" which is not the same as "clean") by passing the water through media like sponge, foam, etc..
With respect to the bacteria/archaea, which consume ammonia/ammonium, produce nitrite which other bacteria/archaea then take up producing nitrate, which is then used by other bacteria and removed via water changes, these bacteria will colonize various surfaces, primarily in the filter, but also elsewhere, and they will multiply to the level sustained by the amount of their available "food." They will not increase beyond this, regardless of the number or size of filters. The substrate is the prime source of the bacteria bed--involving many other species of bacteria beyond nitrifyiers--and the smaller the grain size, the more "space" and effective the filtration. Filtration referring to all types, not just nitrifying.
Plants will grow as well in sand as any other medium in an aquarium, and better in some cases. Here again the grain size can impact plant growth if it is too large for example. Sand is your overall best substrate medium for fish (there are a very few exceptions where larger gravel may suit the species) and plants.