To get the full advantage, put on a header w/ the cat.
Your local muffler shop can make flanges & build a 2" exhaust for about 2 bills, w/o the cat. The Calmini one bolts up to stock or 2" exhaust... sorta.
If you remove/disable your cat, don't *ever* take your car in for inspections or service work. One Midas dealer (in TN) would inspect every car, then "hold it for ransom" until the owner paid a big bill. (Per my brother, who worked there for a while.)