You can go as low as your wastegate is set for.
The boost controller only keeps pressure from going to the wastegate 'fooling it' into staying closed.
example:
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.
Hope that helps
