Nice looking manifold and jigs. Should support big power.
As I mentioned before, the 1.5" sch 10 runners are the exact dimensions of the Steedspeed manifold internally(they are actually ~1.6" inside diameter ports). I do believe you will make more power with this full tubular manifold. Fitting a 6-1 tubular manifold with the 1.5" schedule 10 pipe is a pain due to the bigger bend radius. We use 1.25" sch 10 pipe on most of the e36 manifolds. My car does have 1.5" sch 10 primaries now.
BMW SOB's turbo E30 - 9.20@157
97 turbo M3 - 9.1@163 - first in the 9's
02 turbo M3 - 10.4@135 - Maximum PSI S2 turbo kit