If the biist gauge is connected to the turbo, you will see only boost and that would be the boost at the turbo and not what your engine may be actually getting. You will not see the IC and piping looses. Also the vaccum signal will not be seen at all. You have to connect to the manifold as it is here that you can see both the vacum signal as well as the actual boost you are getting. The point that you specified would be for wastegate connection. You can also use it to get a signal for say extra injection, etc.