You'll gain some higher end power and loose some torque(it's a tradeoff of low-end and high-end power)
your car will be a lot louder and a check engine
light will come on for the O2 sensors.
It's illegal and your car won't pass smog.
I don't see how you could be caught unless they put the car up on a lift, or opened the hood. Which I don't think they could do legally.
They do sell fake catalytic converters to weld in place of your old ones, or you could just gut your old ones to make it pass a visual inspection.
Only do this if you don't have to smog your car every year or more.
I live in rural area in CA, there kinda relaxed in my here. here you smog the car when you sell it or take it off of a non-op. after that you can do what ever you want.