http://en.wikipedia.org/wiki/List_of_Volkswagen_Group_petrol_engines#2.0_TFSI_125-210kW
"The 169 kW (227 HP) and higher versions have
stronger pistons and gudgeon pins, new rings, reinforced connecting rods, new bearings, reinforced cylinder block at the main bearing pedestals and cap, new lightweight aluminium-silicon alloy cylinder head...NOT shared with the lower output variants."
For example, the 2011 Audi TTS uses a 2.0T engine, which is rated at 265 HP.
That engine therefore includes a host of internal strength enhancing upgrades (including a different cylinder block) that the GTIs lack.
Audi does that in order to increase engine life and reliability under the higher boost pressures (17.4 PSIG) that are associated with that engine.
VW/Audi engineers clearly feel that something on the order of 225 HP is about the maximum limit for the GTI's engine when long term reliability is considered.
Boosting your GTI to anything much beyond that level will result in significantly reduced engine life and a significantly higher likelihood of engine failure (in the form of bent bent rods, burnt bearings, etc.)
Even something as "mild" as an APR Stage 1 is going to adversely impact engine life and reliability over time. Anyone who does a Stage 2 or 3 level mod and drives the car hard will likely be looking at some very real (and very expensive) problems by the 50,000 mile mark.
Our engines are designed with the stock power levels (220 HP or so) in mind and apparently don't have a whole lot of room to spare.
More information:
http://www.caranddriver.com/reviews/car/09q2/2010_audi_tts-short_take_road_test
"The engine is an upgraded version of the well-traveled VW/Audi turbocharged, intercooled, and direct-injected 2.0-liter four. In basic TT tune, fed by an IHI turbo delivering 13.1 psi of max boost, it generates 200 horsepower and 207 pound-feet of torque. Enhanced by a beefier block, an aluminum-silicon alloy head, and a heftier crank —and nourished by a bigger BorgWarner turbo blowing 17.4 psi max—this engine churns out 265 horsepower and 258 pound-feet from 2500 to 5000 rpm. "
