taking off your exhaust won't hurt your car one bit.
it'll actually increase Performance (reduces backpressure greatly, obviously) which is good
but it'll blow emissions like a mother, which is bad. and obviously it's illegal because of this.
This will not affect your emissions one bit if you remove the exhaust after the cat, and that's not why it's illegal. It's illegal due to the annoying loud sound that it would produce.
Also, reducing back pressure isn't always good. Reducing it some can prove to be beneficial to Performance, but reducing it too much is just asking for problems.
Back pressure is there for a reason. It's not a mistake that it's there.
When too little back pressure is present then some of the fresh air entering the engine leaves with the exhaust before the exhaust valve closes. So when the engine isn't getting enough air then the computer compensates by decreasing the fuel too.
Therefore, less power.
Back pressure is essential for allowing the correct amount of air to enter and exit the engine when it's supposed to, and on the correct stroke. You don't want Intake
air exiting on the exhaust stroke.