Changing of exhaust does affect your high end horsepower and mid range torque. Just as Mivec Mivec said, if you want a lower noise emission and probably a nice and deep level of sound, get a S-Flow type exhaust. Significant improvement can be felt.
If you are not performing any serious mods, do keep your catalytic converter. Once you remove it, you emission will be smelly, fuel consumption will increase as well. My fuel consumption increased pretty much from 40 liters - 530km to 40 liters - 400km