my previous posting takes into account my personal experiences with split hoses. I've had a split small bore hose coming off of the manifold, a detatched hose from the APS and a cracked intercooler end piece.
In the first two instances the engine ran 'OK' with little or no throttle, but once you moved up the rev range and started applying more throttle the engine would stutter and not pull.
With the cracked intercooler, the engine ran fine but just did not produce any boost.
If you have ever tried to blow into a boost gauge, you will find that atmospheric pressure is actually pretty high, so if you try to apply another 50% pressure (1.5bar abs) even if you have a very small split, most of the pressure will escape out through the split.
If you have a split in a large bore hose, (ie the intercooler pipe) all of the pressure from the turbo will escape out through it, no boost with the APS registering no boost.
However if it is a small bore hose coming off of the manifold, the manifold pressure will not be as high as it should be, but will be above 1 bar abs, but the pressure in the small bore hose at the end after the split, (near the APS sensor) will be far closer to 1Bar than the manifold.
Therefore the engine is getting boost, but the APS is not registering it. Hence the ecu doesn't add enough extra fuel and the engine runs very lean.
This is pretty much my take on the situation, but I'm no expert, if anyone can put me right on any of the points i've made please speak up.
The problem with the guys original problem is that there are so many things that could cause this, you could swap every sensor in the engine and still not cure the problem, it is far easier and faster to take a car with a problem such as this to a fiat/lancia/alfa garage and get the car diagnosed correctly before any major damage is done.