Bit of a grey area, it will void your warrenty on any bits that it effects, which in reality isnt a lot. Any decent dealer should be happy with it TBH, but they could try and use it as a get out clause if something goes wrong, as most will wrongly try and deny any warranty claim because of it. A quick bit of shouting and you'd be ok though
Yah, feel a bit better now. Also googled and found the following and it looks like it would be illegal (breaking EU law "'Block Exemption Regulations 1400/2002'") for Nissan to refuse warranty work as long as the exhaust being used is of equal quality to the stock one.