Benefits of the Tial external waste gate are many! First off the housing is a better design and flows better! The external waste gate does a way better job of controlling the exhaust bypass and in turn is way better in controlling boost!
With just these two examples, you get more consistent boost, quicker responding boost, less backpressure at a given boost ( gets you closer to the optimum theory of one to one, boost vs backpressure theory ) and most importantly better on off the throttle response!
Worth the expense in many ways!
Edit! Think of it this way, if a simple flapper valve was good enough, you would see it in a motor as an exhaust valve. But you don't!