When you fit a ported head and/or cams it is wise if not essential to remap the car or at least confirm the fuel map is correct.

Lets take a cosworth mapped as standard by speed/densisty.

Based on a given rpm and map sensor input (i..e boost) (lets assume WOT) the car will be mapped for the engines actual airflow. This means we are saying that at X rpm and X boost the engine consumes X amount of air and hence requires X amount of fuel. This is fixed by the map in the ecu.

If we now fit a ported big valve head for the same rpm and map sensor output we are flowing more air due to the lower resistance the ports/valves offer to the air. The map will have X injector duration at this point which as it was specced for the std head will now be insufficient for the increased airflow at those same conditions resulting in the engine running lean.

On some engines the head makes little difference if say for example the turbo is the main restriction.

However if you take my engine capable of making 600bhp on 26psi at 7200rpm, imagine the consequence of putting in a normal 403 (grey's) map in. A normal grey map might have an injector on time of 16ms at 26psi@7200rpm and be sufficient to fuel for 380bhp. given my 600bhp potential I would be underfueling by about 35% with disaterous consequences!

so you see the answer is it's not about plenum pressures and rpm, its about actual airflow for a given plenum pressure and thats where ported heads and cams effect things and hence usually need remapping.