Hi mate,
Basically there are a few varients of the 1.8t engine,there are smallport and bigport,smaller and bigger gugeon pins(rods) and forged and non forged pistons and crank.
Basically the early 1.8t had a better flowing head and stronger rods(larger pins) and no vvt.While the later engines and smallport head and weaker rods(smaller pins)and vvt.Only the s3 and 225 tt 1.8t's had forged mahle pistons and crank.Although only the apy/apx early s3/tt engines had the stronger rods and forged pistons and crank but a smallport head.The weak point is the rods in the 1.8t and 320-340lb/t should really be the top mark on standard internals.Power wise ,small pins i reckon 320bhp and larger pins 350hp.The standard s3 pistons/crank are good for well in excess of 400bhp.The largeport head will flow more top end wise but the smallport head will produce better mid-range.
Turbo kits;On standard internals there really is only one choice at the moment which is the jabba/ihi kit which has good response and power.Jbs are talking about releasing a gt28rs lite kit but the ihi is tried and tested and i know of one car that has done 35,000 hard miles(many track) and is still ok on standard internals(large pins).
The gt28rs is a good turbo but is quite laggy on a 1.8t so most people go for the smaller .64 housing but this can only make around 330bhp max.The JBS gt28rs is a hybrid which i believe contains quite a bit of gt30 core hence the big power.This kit definetly requires the rods changing to get the most out of it ,if not pistons aswell.
Another thing that seems to go on 1.8t's is valves ,so it definetly worth uprating these if you go BT.