250bhp will be easy on a standard engine. you can supercharge the standard black top with full system and get 245bhp no problem, seen it done. st170 has better head and vvt so if you make use of that aswell should easily cope with 250bhp.

perhaps if trying to nudge over 250bhp it might be worth using a thicker head gasket to lower the compression ratio a touch. your best off speaking to some tuners who've done it or have tuned the st170 engine before.