Stock iron heads won't cut it. Boost is one thing but boost won't make up for horrible flow. So at minimum you should smooth the flow by porting the heads. But at that point with factory 318 heads I'd find something better.
I would have to disagree somewhat. Forced induction will improve the amount of charge a cylinder head can flow. BUT, the thing to remember is that Boost is only a measure of the resistance to flow between the pump (Be it super or turbo) and the head of the valve. So say you bolt a hairdryer to a bog stock 318, and it makes 10psi of boost. And then you swap the heads for some real sh1thot heads like say some W2's, the boost level would fall to a reduced number due to reduced resistance in the intake tract. This analogy is kinda flawed, due to the fact that the boost figure with a turbo is usually controlled by a wastegate. So although the boost figure might be the same, the volume of charge which enters the chamber will be greater. More charge, means more chemical potential energy and greater thermal expansion and more go!
But I digress. The other side effect of a less restrictive intake tract is due to the charge being compressed by the narrow tract, less heat is generated. Less heat=more density, and more power.
What I'm saying perhaps pertains more to superchargers, but turbo's are still quite similar. I know a bloke who was able to max out his standard injectors, just because to heavy porting. Although the boost level was still at the standard figure.