Stillen upgrades to Nismo?
 

Would these aftermarket parts worth buying, any experiences with these items? Anyone able to tell me an estimated hp gain from these? Maybe spread your knowledge on better alternative brands etc.

Stillen 370Z Ceramic Coated Headers - Nissan performance parts

Stillen 370Z Hi-Flow Catalytic Converters - Nissan performance parts

I have a Fast Intentions CBE and HI flow cats non-res which is a pretty loud setup and has pretty great gains. Tony at F.I has the best customer service by far. If you want a quiter setup you can get also get resonated so they have plenty of options.

I would actually go with long tubes, although those are completely "illegal" by way of no catalytic converter whatsoever

If you've actually done zero mods yet, I'd get a CBE before you think about cats and headers.

Technically speaking, HFC's are also illegal. You cannot remove the factory cats from your car unless they are malfunctioning.