Speak to Mark at Abbey. The Motodyne Y pipe is very good and not silly money either. Sports cats together with the Y pipe + remap will give you good gains without spending a fortune.
Unless you are going racing the manifolds will be a waste of money, also they can crack and become unreliable. One golden rule with regard to power gain figures quoted: ignore them as most are just bullshit. Speak to a proper experienced tuner who knows the 350Z well and can tell you what bits work and what doesn't.
Manifolds will only crack & become unreliable if you buy cheap knock offs, lets not make generalisations here Manifolds will make extra power - I know this because my car made more with them