Well, I can't comment on Cossie gay boost control, but take my setup for example. My 1.5 bar spring results in a minimum pressure of around 1.4-1.45 bar of boost. I can then use a valve to allow more boost on top of that. So, valve off is 1.4 bar, valve at around 40% duty makes around 1.8 bar.

If my wastegate were sprung to 1.0 bar, it might take 80% duty on the valve to achieve 1.8 bar and the more you ask the valve to do, the more it can become unstable.

Thats my take on it.