Well I couldn't get the RB20 elbow doesn't fit the RB25 turbo. I just wanted something that would bolt on with no hassles. But the inner diameter of the elbows were different too, so yeah, the flow would be all messed up.
The elbows dont go for much anyway, there's one for sale right now for $50.
Oh yeah, and another thing, you need to get a pressure source to hook up to your wastegate actuator. I just got a nipple tapped into my elbow, some do it on the compressor housing.