You could be leaking boost through the PCV system, with as much that is invested into this car, I would add a AOS (Air Oil Separator) Perrin, Grimmspeed and Crawford are the top sellers.
You could also have a tear in one of the couplers, I've seen that countless times.
And what is the definition of "passing" a boost test? Do you mean boost leak test? And if so, you either leak or you don't, there really isn't a pass or fail margin when boost leak testing.
Most of the time it usually is something really small and overlooked, could be anything from a leaky vacuum hose, turbo inlet hose, to the actual charge piping, anything that sees a boost reference, or signal from boost pressure has the chance to leak.
In Evo's the most common boost leak is the throttle body shaft seals, I change them out on the daily on Evo 8's and 9's. With most Subaru's I've found it to be a vacuum line or the PCV system. But the fact that you have a front mount intercooler setup, you could have a small tear or pin hole in a silicone coupler, that would leak boost pressure like a three day old diaper.
