Exhaust

Stock exhaust system is 2.25" and most aftermarket exhausts start at 2.5". On a stock PI GT you won't get much of a HP gain changing to a 2.5" exhaust unless you go with something like long-tubes. Just want to manage expectations here. You can drop $1K on a nice mid-pipe and cat-back exhaust and you might get 5-10HP on the dyno on a stock PI4.6.

The cats arent as restrictive as everyone thinks. Again, on a stockish car, you might get 3-5HP if you remove them.

Most folks change the exhaust for sound. The stock-cat back is pretty quiet. Leaving the stock mid-pipe alone and just changing the cat-back might get you to where you want to be without ditching the cats. Or it might not and you can explore changing the mid-pipe out after. Some folks like mild, and some like it very loud. You will need MIL eliminators to avoid the check engine light as eliminating the cats will cause the CEL to come on. You will not pass a state OBD check as monitors will not be complete with them, so if you need to pass an OBD test, you might not be able to without cats.

Cats are federally controlled emissions equipment, so it's illegal to remove in all 50 states. However, not all states inspect and enforce them, which is why some states it's easier to get around these laws, and some like CA it's impossible.

If you do decide to remove the mid-pipe, keep it. If you sell the car, it might be easier to sell with the stock pipe if your state requires emissions testing. Those pipes are getting harder to find because everyone pulls them off and scraps them

What crazy stuff?