Yes the snell is a snell rating. Do you want the the minimum that meets requirements or would you want the one the exceeds the requirements?
You can put a brain bucket on your head if you want.. I prefer to spend the extra money and buy a known quality part and not have to take a chance on failure. I don't have a $10 head so I don't want a $10 helmet.
Using you analogy a broke rocker arm at worse will kill the motor. A broke helmet will kill you. Take a chance on the motor not your head.
