A good condition Cat only provides around 15% of the total restriction in a exhaust, the other 85% is the pipe size, mufflers, bends etc.
So lets say you run open headers and gain 30hp, only 4.5hp of that would be attributed to removing the cat.
Considering the amount of good the Cat does, and the fines associated with not having one, the small gain in Hp you may get isnt really worth it
There are easier ways to make Hp and still be street legal....