External gates arent always consistant, just larger so they dont usually cause boost spiking.
The issue you will usually see with boost tapering off is the wastegate blowing open from the massive pressure in the manifold, sometimes up around 60psi or more. Most wastegates will leak at those pressures. Added to that is the engine is reving more, and swallowing more air, requiring the turbo to work harder to compress the air. Eventually it wont be able to flow any more and boost will start to drop. Remember that boost is only a measure of restriction.
It does make it hard to tune around but as long ars these problems are consistant the tune will be ok. If you are worried, ask them to back it off a bit. (even NA's knock up hills though). Or you can fill on ethanol and never worry about pinking again.