This analogy gets used a lot, but it's a good one: whenever you put good fuel in an engine, it's going to run better. If your car is supposed to run on Premium, and you put cheap, Regular or Leaded fuel in the tank, it'll run, but not very well. It may cough, sputter, stall, or just not get the gas mileage that it should. Put the right fuel in the tank, and it will run the way it was built to.
Your cats are now getting "premium" fuel in their tanks and they're running the way nature built them to. It's just that you've lived with the coughs, sputters and knocks for so long that they seemed "normal" and it's strange to see the engines running smoothly.
Congratulations on improving the "engine performance" of your furkids!