Nice test but as Clark mentioned the intake manifold isn't the bottle neck at those power levels so it's no surprise it's just a pretty (and expensive) intake manifold on that application.
I did tune one recently on with a full Cosworth long block and it's clear this manifold doesn't show it's worth until after 6k rpm. This feeling was backed up with the Hydra EMS as you could see the dramatic fuel increases above 6k rpm which remained flat until 8000 rpm. Other problems with this particular car have prevented me from dyno tuning it yet but it will be interesting to see how it compares to the Subaru (non STi) manifold I run on our Time Attack STi.
Comparing the Subaru RS manifold I run to the stock STi manifold there is a loss of bottom end for a gain from 5500 rpm to redline. It will be interesting to see if the Cosworth manifold has any gains over a much more affortable Subaru RS manifold. At least neither will crack