What's the condition of your intercooler like? Are a lot of the vanes bent and damaged? Is the intercooler full of silt? What about the condition of the turbo hoses?
Does the vehicle use much oil? Is it possable that the cam is badly worn?
What about your inlet manifold? When I took mine off it was full of crud, proberly restricted by about 20%. Also is the manifold gasket on it's way out?