For the gate to open, the spring and actuator pressure has to be defeated.
An increase in backpressure will directly contribute to press the gate shut, from the back of it.
The pressure in the up-pipe is always higher, but that's why we have a spring and an actuator that converts compressor pressure into closing force.
The flow of gas has a static component, where gas is just stored inside the turbo, behind the wastegate. The position of the wastegate can be looked at in terms of pressures.
But then the flow through the turbine determines the momentary pressure in the uppipe.
The gases pass through the turbine, lose energy and slow down. They exit at a flow rate that has to do with the turbine parameters. For the pressure in the uppipe to be increased, this flow would have to be affected severely.
Another thing is that APS had backpressure test results on their page and those numbers were not really big, even for exhausts that were below 3".
This is also noticed by people that upgrade from 2.5 to 3" and don't report much of a change.
So, while I'm sure there is an explanation, I'd like to get more details.
I think that this has to do with what happens in the turbine at compressor boost threshold.