I understand your question and its a very valid one. This extra turbine house is however meant for a small series of same-size frames.
Most turbo's can be ordered with a variety of turbine A/R's so they all have the same 'form' so to speak.
After some wrenching today it turned out I had an exhaust leak from the complex part where I made the twin exits into one for the wastegate.
I have welded most of it shut now (saw some black traces) and had full boost again
Not the same respons I had on my trip with this housing to germany last week, but close enough. Maybe I missed a spot, will have to look lateron some more into it.
Pfft..