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This is a fairly common problem with turbo cars in general. To understand why you first must fully understand all the pressures/forces involved in making your wastegate function. Most people think boost pressure vs actuator spring force is all that's going on... But that's only half the story. You also have to consider the force applied to the wastegate valve/flap by the back pressure in your exhaust manifold. The actuator spring will open the wastegate when the sum of these two forces exceeds the spring force.
When you increase the flow capacity of your exhaust system you decrease the pressure on the wastegate valve, meaning more boost pressure is required to reach the same net force required to compress the spring... So your boost creeps.
The only way to remedy this condition is with a lighter actuator spring. Or, I suppose, going back to a more restrictive exhaust, but who wants that? ☺️
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