that fault isn't going to give you an overboost error. that part is on the wrong side of the turbo, so air in that part is not charged by the turbo yet, so its not under positive pressure at least!
it is possible that it could give you are error as you will be getting unmeasured air through the system which could make it run lean or whatever but I wouldn't count on it being the root cause of an overboost error.
at the end of the day the car should regulate its own pressure using the VNT system, so the actuator should control the turbo so that it provides enough boost to match the desired vs actual boost level. an air leak wont help this and does need to be repaired but I cant see it being the root cause.
typical overboost fault is caused by the actuator arm on the turbo being stuck, bit of grease helps here.
a faulty map sensor, try cleaning it before you replace it.
split vacuum hose, cheap enough to replace either side of solenoid
faulty solenoid / actuator for the VNT
they are the typical ones.
you should be alright probably up to 3rd gear, hard acceleration at 4th and above will definately produce max boost and trip the error.
where abouts in the rev range are you overboosting? is it a constant overboost or is it only overboosting above a certain rpm?
when I had the error, and it is most frustrating trying to fix, it turned out to be the VNT actuator / solenoid that sits on the radiator.
best of luck, hope it helps!
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso