Not quite sure I understand the point you're trying to make, because getting back to the original question, though, "Are modern cats even that good at killing rats anymore?" - the answer is yes, absolutely.

There is no real genetic difference, as a total population, between feral cats and cats owned as pets. Absolutely, as you point out, owned cats who know they'll get fed inside will murder outdoor animals just for fun, and they don't need to eat them to survive - but that's a learned behavior, nothing they were bred for. You take a normal tabby and leave it to have its litter outside, unowned, and those cats will turn into feral cats.

Many animal rescue organizations have developed "Barn cat programs" for feral cats. These cats have not been socialized to be around humans so they make bad pets, but they're happy to live in a barn and ensure it says rat-free: https://www.austinpetsalive.org/programs/barn-cat-program.