Not entirely sure on how the boost is controlled on the 2.5, but, this is the place to begin. Does the turbo have sufficient output and what's controlling that output?

Doesn't that vehicle have a vacuum controlled wastegate? If so, the problem is almost certainly due to the vacuum control system. There's a vacuum leak somewhere or the wastegate itself is not functioning properly.

If the ALDA doesn't see boost, the vehicle will always be a dog.