You can only get 2X the boost pressure produced by the spring alone, by adding a boost controller. I've seen it in practice, and it's a generally known rule of thumb.
Supra owners with large single turbos typically run an 18PSI spring in their EWG, in order to be able to hit over 30PSI on their race gas setting.
The reason that the wastegate starts opening before the preset boost level with an MBC is inherent to the way an MBC works. You are using a spring to hold the gate shut on one side, and restricting the air that is pushing against the other side of the spring in order to hold boost higher than the spring rating. It takes a certain amount of force to push open the gate against the spring pressure, and in order to exert this force, the MBC gradually allows more and more of the air signal through, in order to have the gate open at the prescribed boost level.
An EBC works in a completely different manner, by adding air to the same side of the wastegate that the spring is on, to help hold the wastegate shut against the air coming in through the side port, that is pushing against the spring. This means that once you are running more boost than the spring alone supports, the valve is operating on a pressure differential (like the stock and HKS BOV's do). With the pressure differential, the wastegate stays closed until almost the preset amount of boost is reached, because it only takes a small amount of extra pressure on the bottom to pop the valve wide open, and that can happen very quickly, compared to pushing it open against spring pressure.
Ken