A gradual decline of power could point to a few things, but i'll leave the more dramatic stuff (bad injectors, loss of compression) out of the picture because of the relatively low mileage of the engine and the gradual effect. Things that are most common in these situations:
- Engine fouling: Could be a factor especially when a car is driven economically and does not see a lot of high speed or highway use. Soot buildup in the exhaust manifold, turbocharger and possibly EGR valve robs the engine of power.
- Boost leak in air intake. When a hose, hose clamp or intercooler leaks air you will lose some of the turbo boost pressure and therefore power.
Both of the issues can be diagnosed at a service station. An engine overhaul sounds unlikely because of the relatively low mileage and a worn turbocharger leaks oil, causing to a lot of white/blue smoke from the exhaust. If that is not the case than the turbocharger probably is ok, apart from possible fouling ofcourse.
