That was exactly what I was thinking. If the lack of power is rev-related, it can be caused by the fuel-filters...
If the lack of power is speed related, it will become more difficult to find a cause.
Ben, I don't know if you got anything about the history of the car, but I think that I would start with renewing the fuel filters. The lack of power could be caused by a turbo problem as well, but then again: it has to be rev-related.
with a minor 600+k