Basically the car came with 2 inch intercooler pipes and 2 inch exhaust pipes. Both exhaust and intercooler pipes are too small for even stock turbos. The system is only as good as the weakest link or smallest restriction. If you install a larger turbo, you are really restricting what it can flow by leaving the stock 2 inch exhaust on. The same goes for the intercooler pipe. Everything needs to flow more volume of air for the system to be efficient. Either side having a restriction will cause the turbo to work harder to move the air and it will cause the turbo to spin up slower and limit the speed of the turbine. Think of a garden hose vs. a fire hose. No matter how high you turn the pressure, only so much water is going to flow through the garden hose. The fire hose even at low pressure is going to move a huge volume of water and as you turn the pressure up with a fire hose the amount it can flow is insane. Same goes for the intercooler pipes moving air. The larger volume of air they can move is huge for power.
