[this is not cbring]

the stock turbo is a KKK (it is more commonly referred to in this country as a K03)

personally, i wouldn't turn up the boost in otherwise completely stock trim, heat is a real issue unless you go with an exhaust that is more free-flowing..........also the stock diverter valve is plastic and can't even handle the stock boost for long, but for about $150 you can get a DV that'll handle all the boost you can throw at it

however, i seen many set the boost at 1.1 bar (roughly 16 psi, i think)on an otherwise stock car and have no problems other than blowing out a diverter valve

with bolt-ons and a computer program, i've seen many a people run 1.3 bar (19 psi) and stay fairly reliable

as far as how much the engine can handle........i've never really heard of anyone with any streetable setup having such problems.......volkswagen has been using that 1.8 block for 20 plus years now and it has a great reputation

as far as the boost controller, i don't really know much about manual boost controller setups.........must veedub guys use and recommend a "chip" to reprogram the ecu.........the computers on these are pretty sensitive and i imagine turning up the boost without doing much to air/fuel ratios will throw a code real quick