You will not gain anything from removing the cat from a P10 or P11. Nissan cats are well designed and have a very large inner diameter (3" I believe). The amount of backpressure a factory Nissan cat makes is so minimal, it is not worth the time. Also, your exhaust will get much louder. (Personal experience on my old '91 NX2000 - bad idea - quickly replaced the cat)
Go here for more info:
http://www.sentra.net/tech/garage/en...aust%20Systems