300 HP out of a 318, is it possible with bolt ons?

why?

you're not machining anything. you're not modifying anything in the basic architecture of the engine and you don't need any specialty tools (beyond a dampener puller) to install one.
I know, "bolt on " is a pretty loose term. Where does it start and where does it stop. If there's a competition going on, the limits need to be mentioned.