Maybe I am missing the point completely (please set me straight in this case) but I was under the impression that internal gates struggled to stay closed at full boost due to their size, something which is ultimately determined by the size of the turbine housing.

Christ, a number of factory external wastegates struggle to stay closed at higher RPM unless going for a refurbed unit (if they are old) or a dual port wastegate (this was my experience and others on a previous turbocharged car I owned).