There are four different 1.8T motors that were used in the TT, by engine code they are:
In the 180 - ATC, AWP
In the 225 - AMU, BEA
Here's some facts pertaining to high performance applications of these engines:
All 1.8T TTs have a forged crank and pistons.
Yes, all TTs, right down to the FWD have forged internals (minus the rods). The stock crank has yet to break from a high HP applicatoin, same with the pistons; they are made by Mahle and have withstood over 700 bhp. The part prone to failure is the rods, which is discussed below.
Rods in 1.8T TTs vary in size, but some 180 and 225 models have the same rods.
The ATC engine, used in model year 2000 TTs, uses 20mm wrist pin rods, which makes its internals identical (strength wise) to those used in both 225 motors. However, in 2001, Audi started using the AWP motor which does in fact have weaker 19mm rods. The rods shouldn't be trusted for applications above roughly 300 ft-lbs of torque.
The 225 has a "webbed" block...but nobody has broken a 180 block yet anyway.
Audi included some "webbing" around the 225 block to increase its strength. However, at least in the short term, someone has yet to cause the 180 block to fail.
Some TTs have wideband oxygen sensors, some have narrow band.
Early TTs did not use a wideband oxygen sensor and later models did. Not all 225s have wideband; the AMU 225 motor, used until somewhere in 2003, still uses narrowband. The AWP 180 motor, used in TTs since 2001, incorporates wideband. In a nutshell, ATC 180s and AMU 225s don't have wideband, AWP 180s and BEA 225s do have wideband.
Wideband is a highly desirable option for tuning, since it allows the engine to precisely control air fuel ratios to the point where the engine can run without a MAF sensor, given the proper ECU software.
The 180 and 225 have different compression ratios.
The 180hp TT has 9.5:1 compression whereas the 225hp TT has 9.0:1.
225s have bigger injectors
225s have 380cc injectors whereas 180s have 318cc. For almost any big turbo setup, however, injectors will have to be swapped out.
Some TTs have variable valve timing, but it's not for performance.
VVT is used on startup for emissions and makes for ridiculously expensive cam chain tensioners.
There are other differences between the 180 and 225, primarily related to the turbo hardware.