Here you go Darren a quick run down on how actuators work.
As you know the actuator is connected to a vacuum line that runs from your inlet manifold, inside it has a diaphagm which compresses a pre tensioned spring. If the spring is 10psi (and pretensioned correctly) it will take 10psi of pressure from the manifold to compress the spring. This in turn will push the wastegate open and divert exhaust gasses past the turbine wheel and your boost will start to drop, when it drops below 10psi (@manifold) the wastegate will close. This will carry on opening and closing regulating your boost to 10psi.
Unfortunatley this is quite a crude (but effective) method of boost control that works better on some setups rather than others.
Now we'll talk about back pressure, as has been established boost is a measurement of air flow restriction, therefore the turbo has become a restriction to exhasut gasses and will cause back pressure between the turbine and cylinder head. Now this is inherently worse on systems using log manifolds such as whats fitted to our clios and obviously becomes greater the higher up the rev range (more airflow=more back pressure) you go.
Now all this back pressure will be pressing against the wastegate located in the turbine housing. So imagine if we have say 3psi of pressure pressing hard against the wastegate this means that it will only take 7psi of manifold pressure to compress the spring in the actuator. If the back pressure was 5psi, it would only take 5psi of manifold presure and so on and so on.
So when you put your foot down in your car the boost will hit its peak and as the back pressure builds in the turbo manifold it gradually decreses the amount of intake manifold pressure that is required to open the wastegate, this is most commonly known as boost creep.
So how do we get round this? Well we fit a fancy boost controller. What this does is use the inlet manifold pressure as a reference source and blocks any boost pressure from reaching the actuator. This in theory (if still using a 10 psi actuator) would require 10psi of back pressure to force the wastegate open and should therefore allow you to run any boost setting you want.
Go back 2 months and my boost control set up consisted of a 1bar forge actuator, gizzmo ms ibc and guess what? Still mega boost drop LOL so don't get your hopes up that the 10psi actuator is going to change things to much (unless your current actuator is only 2psi and the back pressure is blowing it open)
Hope this helps mate, and if anyone spots anything I've missed feel free to add it.
Steve