your might be better off asking on the FFOC.co.uk forums, some on there know all there is to know about the ST170's and its limits. would need to know what power and PSI you want to put through it.

also www.focal-jet.com (a USA focus site) has zillions of turboed and supercharged focuses on there, they will know the limits of the pistons (their SVT focus is exactly same as ST170)