The boost controller only keeps pressure from going to the wastegate 'fooling it' into staying closed.
You have a 14psi wastegate but want to boost 20. The controller sees 14+, but does not open the valve until it sees 20(he set amount). At that point the wastegate will see the 14psi and open like normally.
If you turn a boost controller to zero, it will open the valve to the wastegate at the moment you make any boost, but the spring on the gate still holds until 14psi before it opens.
if you have the spring that comes with the t78 kit in the wastegate, then you need to get the "yellow" wastegate spring to run lower boost. the spring that comes with the kit will let you run a minimum of i think 1.0 or 1.1 kg/cm. if you are running stock fuel dont use a boost controller. thats just another variable that could malfunction and cause overboost.