His set-up could be twin-turbo, or he could be running a divided T4 set-up. I would recommend running the boost gauge vacuum line from your compressor housing. I would also recommend running your WG's/boost controller vacuum line from your intake manifold. You will have a much cleaner vacuum source when running it from your intake manifold. Ideally, you would want to run all your vacuum sources from the intake manifold, but I know you had mentioned you cannot do that. Other than this, I highly recommend a boost leak test. Does your boost gauge needle flutter when you are in negative vacuum?