If your car is underpowered, or going into limp mode, you may have a split intercooler hose. The hose in question leads from the intercooler to the EGR valve and is accessible on the front left of the engine manifold. To find out if the hose is split feel around the back of the hose. If there's a crack the engine will be losing turbo pressure, hence why your car is underpowered. If there's a huge hole in the hose your car will go into limp mode and the heating coil symbol will light on the dash. It'll still run but without the turbo. Very slow!
The problem is that any oil getting on the inside of the intercooler hose deteriorates and weakens the rubber. And eventually it forms a crack, which gets bigger and bigger over time. If your car makes an 'air rushing' sound on acceleration, that is the pressurised air (and thus horsepower) escaping from the hose.
Obviously oil in the air system causing these problems would indicate another fault but I wouldn't know about that, or how to deal with it.
I had this lack of power problem. I bought a replacement hose and fitted it myself, It's quite fiddly getting to the hose clip at the intercooler, but otherwise you just need a few tools which most socket sets will have.
The new hose I bought from ebay lasted about 3 months before it also let go and split. I've recently bought a slightly more expensive silicon hose hoping that any oil in the system won't deteriorate it. So far so good. The power of the engine is so good.