Yes you can cut off or remove the catalytic converter from your vehicle and replace it with a straight pipe as long as your state doesn't have any emissions testing.
If you're in a state that requires emissions testing then you'll likely have your vehicle fail the emissions testing and if the inspector sees your vehicle has no catalytic converter they may request that you put a new catalytic converter back on the vehicle before it passes the emissions testing and inspection.
Removing the catalytic converter will increase horsepower and make your engine more efficient.
It's not against the law or illegal to actually remove a catalytic converter from your vehicle and you usually won't get caught when stopped without a catalytic converter on your car nor get into trouble for not having the catalytic converter on your vehicle.
However if your state requires emissions testing on your vehicle then your vehicle may not pass the emission testing and then if the person inspecting your vehicle notices that you've removed the catalytic converter then you could possibly get a fine of $2,000.00 to $10,000.00
But in states that do not have emissions testings removing the catalytic converter is perfectly fine and you won't get into trouble nor will anyone know about the catalytic converter being removed.
I removed my catalytic converter from my truck and my horsepower has increased as a result and I would not want one on my vehicle anyway.
But if I were in a state that had laws against removing the catalytic converter or emissions testing then I would leave it on the vehicle.