timing is everything on honda motors.....depending on how fast the revs come will determine it. making 250 hp may not break a valve or valvespring but will definetly wear it out a lot faster. all it takes is one worn valve or spring.....and im not so sure about a 400 hp stock valvetrain si....if someone is pulling this off it must be really well tuned or something is gonna go wrong. that seems a little out there. you're creating a lot more pressure in the cylinders with a turbo so the valve is under a lot more stress with each opening.....they're not designed to handle the pressures boost brings and thats when they get worn