[minimutly]

Entitled to your opinion ColinNI, in fact i don't think you're far off the truth about "tuned" versions just not getting the power they should. My experience with them is that the std cylinder head is restrictive, the turbo too small, and the std ecu not up to power increases. That doesn't mean that this little lot can't be fixed, and for little money:-
Cylinder head - ported numerous normally aspirated ones, has a huge effect.
Turbo - I have a subaru impretza 300 bhp one here ready.
The zt ecu will do a sterling job, with a remap and larger injectors - it won't know the engine is a cast iron blocked 600bhp capable pensioner.
Other stuff may and no doubt will need checking and or changing as required, but 250 bhp should be possible, reliably.
And lets not go into shafts and the rest of the drivetrain, that's up to the way I treat it.
As for the vag engine, I think you'll find the earlier verion was both stronger and simpler - strange that, how an older engine was better than the later version....