Typically we see horsepower gains in the top end of the cars performance. The stock internal gate on subarus are weak at controlling boost in high load areas, the gate itself is too small and gets forced open and boost pressure is lost up top. We can remedy this problem by permanently closing the internal wastegate, via a bracket such as the one grimmspeed makes.
Another option is that we can permanently weld the internal wastegate bracket shut. At this point we install an up-pipe with a seperate section that is has a 38/40mm v band flange, or a 44/45mm v band flange. This allows us to install an external wastegate which has a larger diameter, which gives more flow control of the boost pressure. A couple great options are the turbosmart comp gate 40’s, hyper gate 45’s or something comparable from Tial like an MVR.
This is necessary because when we start increasing horsepower and boost pressure over stock levels the internal wastegate was never designed to hold back excess of 15psi of boost.
This combo is what we see fit as a best step forward from the stock setup. Actual horsepower gains vary, due to things like turbos being run, actual boost pressure, the way the tune is setup, etc. But it is said that for every 1psi of boost pressure there is roughly 10whp to be made. Typically the internal wastegates cause subarus to lose up to 5-6psi of pressure near redline. We can see as much as a 50whp gain under the curve near redline. That is a lot of lost top end power, which we all know that these Subarus need!