I am not by any means saying I have the answer, but it is entirely possible that the nipple you used is at a point in the manifold that shows lower pressure. This could be due to a combination of the flow rate and turbulence in the air stream. If it would not be too difficult, you could tap into a different place and see if it makes any difference.
Also, what does Speedparts list for boost for your car? I would hope they can give you some kind of info on the boost pressure it is supposed to make.
Also, what does Speedparts list for boost for your car? I would hope they can give you some kind of info on the boost pressure it is supposed to make.