Check everything on this list.. then report back..
There are several reasons in general why a turbo charged car doesn't perform as it should : (i've owned previously quite a few turbo cars..)
Assuming the loss of performance isn't environmental (high temps, high altitude) and it isn't poor fuel :
1) Vacuum control lines leaking or fractured : result - turbo wastegate stays open.. only running base boost setting. Fix - Replace all old vacuum lines with new silicone lines (or OEM... whatever).
2) Wastegate actuator is leaking under boost : result - turbo wastegate stays open.. only running base boost setting. Fix - Replace wastegate actuator.
3) Dump-valve (diverter valve) is leaking (diaphragm burst) : result - engine can't lock down boost.. so boost is vented away. Fix - Replace Dump Valve.
4) Hole in the intercooler delivery pipes. : Result, air being metered is wrong.. running rich, bad idle (at worse case), smoke on acceleration.. no power.. maybe even limp home mode. Fix : Replace pipeing
5) Boost Control solenoid problem : Motor management instructs valves to open and close to control the wastegate, this is usually done through a solenoid.. can become faulty in cars older than 3 years. Result : Wastegate defaults to open and vents boost. Fix : Replace the solenoid.
6) Air Mass Meter failure : Very common failure point on a great many cars.. air not getting metered properly.. shows up usually is a huge flat spot where peak boost ( and peak torque shows up on the map..).. on an RS2.. peak torque of 410NM is at 3000rpm... so I'd expect a feeling of surging on/off acceleration around there if the AMM is faulty. Result : Car is metering and measuring intake air all wrong.. it's adjusting boost and fueling as a result... and getting it wrong for the combustion process.. Fix : replace air mass meter.
7) Lambda sensor failure.. causing a loop back failure on the fueling/air/boost mix.. Fix: Replace sensor in downpipe.
Now getting into the expensive reasons :
Fueling problem.. perhaps injectors are faulty/dirty.
9) Head gasket problem.
10) General engine issue
11) Turbo seals
12) General turbo turbine failure