Ok,
I am currently running a non divided 22 cm BHT3B, its a Garrett and "they" say that it is suppose to be 7% more efficient then its HT3B counterpart.
To keep with the same drivability I would like to stay with a relatively small turbine housing on a GT42. So for comparison sake lets say the 1.15 a/r is close in size to a 22cm?
Also the GT42 is available with a T06 flange. As Brandon pointed out it would be a smoother transition from the hot pipe to flange.
Not to mention you can get a waste gated housing for it!!!