Answered your own question there. S15 is an older design, and designed to older regs. The RX8 is more comparable, but smaller engine, tank, brakes, and being physically smaller soon make up 200kg.
Porches weight is of course going to be much less when you look at the retail prices!
370Z is a shade lighter according to my unscientific look at the back of EVO.
Carbon seats and a Ti exhaust would soon lose you 50Kg+ from a 350Z.