As donny said, the SPI is a strong motor, if you replace the head to prevent the valve seat drop. I have seen several successful SPI turbo projects. It is not the easiest motor to boost, because you can't just buy a kit and bolt it on like you can with some other motors. but it can and has been done. I have heard that when done correctly the SPI can handle more boost than the zetec because of the different blocks. but that is information I recall reading 7+ years ago, so it may be incorrect.
don't listen to people like zxthrizzle who don't have any idea what they are talking about. they just make blanket generalizations based on a small amount of information.
Do you own research and make your own decisions.
Defending the reputation of the red-headed step-child of the focus world for far too long.