That will reduce in older version's.

However casual players dosent have enough experience of the game.
Spawner spawns mobs in an 8x8x3 around it. Usually it will spawn 4 mobs from 10 to 40 seconds within 16 blocks of the spawner. As i've seen so far all the grinders are with fall dmg, no cactus, or mob transportation to the killing chamber.

There are several item transportation and way cheaper from the hoppers, u need only 3 hoppers where the mob will die, connect it with dropper , drop the items into the water tunnels with some ice under it so they can travel faster, and dircetly above to the storage system u have. I've connnected 4 grinders into one, 1.000+ blocks long item transportation with item elevator directly to the storage system i have.