not sure about the stock boost, but there are 4 limits with turbos:
1) the engine compression ratio - there is a limit where knock with happen at any ignition setting - at this point you need to reduce the compression ratio of the engine to use higher boost
2) block and rod strength - above a certain torque (depends directly on boost) then the rods will break/bend or the block will be damaged
3) the intercoolers are usually crimped at the ends and tend to unwrap and explode above a certain pressure - most production cars have trouble above about 1.5 bar but this is only a rule of thumb
4) turbos produce more torque in the mid revs range, and their efficiency drops off higher up the revs range - outside of their optimum band they cant maintain boost - you get a great whallop of boost from 2500-3000 upwards until 4500-5000rpm then it trails off quite rapidly
if you set the boost very high, you get a big surge in the mid revs - most turbos can generate enough boost in the mid range to bend rods or blow up engines but then fade rapidly up the revs scale - from a practical point of view you use a boost pressure that is reasonably well maintained to give a good working revs range or you have to drop gears about 5k to 5.5k revs, and may go off boost a bit in the next gear.
the 1.8t engines are commonly run at between 1.35 and 1.4 bar generally but with smaller turbos the torque/boost will fade rapidly above 5.5k revs, and as much as 1.8 bar occasionally peak in the mid revs, although the intercoolers may go leaky above 1.4bar according to jabba.