A few weeks back I found that my turbo actuator wasnt modulating properly. I have since fitted a new turbo valve and a bit of vac piping and all is sunny in the land of GT.
To diagnose I by-passed the turbo valve and put the vac hose straight from the manifold on to the actuator, this meant it was always at max but over 2k revs it would fault on overboost. By doing this though I was amazed to find that I could use top gear at much lower revs, something I hadnt previously been able to do.
With the new valve the car drives better than it ever has through the gears and power is amazing.
The most significant thing though is that the fuel consumption has gone from 35mpg to 43mpg.
In the past I have replaced turbo hoses, new thermostat , fixed calipers replaced air intake hose, air filter etc etc but nothing else I have done to the car has made anything like as much difference.