The SiR head will turn your car into an LsVtec and will add anywhere from 30-40 horsepower on a stock LS bottom end. It is not a simple procedure however, and certainly wouldn't recommend turboing aswell as using stock internals on an LsVtec. You would need an ECU as well, more than likely reprogrammed (jump from 1.6 litres to 1.8).
It is quite a common upgrade people have been doing for some time now. Just search for LsVtec on google or around on this board.