| A custom Map or tailored map will always be the best solution.
To gain any performance increase more air (oxygen ) must be passed through the engine,however this is done pulley/head /manifold etc fuel must be altered to suit .
Any air increasing mechanical addition say a head change is non linear so the fuel changes are also non linear and the map should compliment the mechanical change.
The cylinder head is major change in the airflow to the point I dont personally consider the engine tuned at all without a modified one!!! It is costly but by changing everything but the head is dancing around the heart of the tuning concept.
This is one reason we offer the Unichip with a selection of maps available to suit various combinations of mechanical changes.
As a stand alone alteration ,the map cannot add more air and so cannot increase performance by any significant amount,however the more you change the mechanicals of the engine the more a custom map is required to maximise the benefits of getting more air through the engine in the first place.  | 
