I've found each stove has a sweet spot and burning time, that will yield "coal to dust powder" result. coal quality also is key.
really large firepots required a higher temp and/or longer burning time, to get that result. smaller firepots would do it easier because there's less to burn, so it's "easier" for the stove to burn it to fine powder. the larger firepots give the longest burn time, there's more reserve fuel in the firebox.
there's only so much btu's in each lb. of coal, so no stove can extract more than nature put in to begin with. some just take a higher heat setting to burn it all up to the edges of the firepot, those are typically the really big stoves, or a furnace. dampen those off and there's unburned coal left around the edges, as the fire tends to go into the middle. the round pot will tend to burn coal more evenly, there's no corners for coal to get stuck in unburned. but I've seen both round and square fireboxes do equally well. those old kitchen cook stoves had square fireboxes, but baseheater-like passages, those buggers would heat as well as any parlor stove. there were quite a few old timers heating the entire house with kitchen stoves, with square or rectangular firebox !