The reason to swap out the pistons, isnt necessarily to drop the compression on a boosted motor. You have to take into consideration a couple of factors.

You have to decide what you want to do with the car. Are you building it to be an all out HP monster, or do you want to work within the stock parameters?(ie: do you want to leave the fuel system/ignition in stock form or modify them?)

If you are looking to make a power increase but nothing too crazy and want the reliablity of a built motor, then swap out the stock slugs for a set of forged units and leave the compression at the stock level. Remember that most of the aftermarket TT kits are built around the stock compression levels, so you can run recommended boost levels the manufacters list, or go a bit higher, knowing you have built motor.

Remember though, to start increasing boost in our cars, you have to be leary of the fuel system. Being a returnless fuel system, the motors rely on a voltage draw from the pump that regulates flow from the pump, and once you start to up the boost in a VQ, the stock pump will start to drop in pressure b/c it cannot flow the voltage need to supply the demand of the pump.

anyways i can keep going on, but ill shut and leave you with this. We have a local greddy TT powered 350z thats putting down 375 at the factory boost setting and its tuned on a reworked emanage ultimate that is done at the local tuner shop owned by my good friend and teamate. The car has run flawlessly with no issues for the past year on stock guts and pump. Now he is considering upping the boost but before we tear the motor out to be sent out and built, we are going to set him up with the APS fuel system and see just how far we can go before we REALLY need to go the route of a built motor.

These motors are stronger than we all think. We just need to take baby steps before taking a giant leap.