As you start to modify you will see that raising boost is one of the many key areas to an increase in power,raising fuel is cruitial too. The standard fuel pumps are known to not be sufficient at pumping enough fuel.
As you raise boost,you require more fuel,bigger imnjectors and so it goes on.
Now thats a very basic explanation.

Scott
If you would care to give a more techy answer i am sure it would help.