In my limited experience tuning Subarus, I have seen "boost creep" manifest itself in the following ways:

1. True Boost Creep.
A slow & steady climb of manifold pressure related directly to engine rpm and is not corrected with recalibrating the ecm. This results in a hp curve that looks like a 45 degree ramp.
The #1 cause that I have seen is due to the restrictor pill being removed/ wrong size restrictor pill. In this case, porting the turbine housing waste gate will not help.

2. "Boost Creep."
Manifold pressure increases normally, then rapidly oscillates by ~5psi. I have seen this exclusively on vf34 turbos. Not saying that all 34s do this, just that all cases that I have dealt with have been 34s. All cases have been corrected with light wastegate porting.

Now on to overboost fuel cut.

The classic example of OBFC is experienced by the driver after removing the factory cat section of the exhaust and not recalibrating the ECM.

Removing the cats allows the velocity of the exhaust gas flow to change quicker than with cats
This allows the turbo to change velocity/pressure faster then the ECM was programmed for.

Since the ecm can't effectively control boost, the rate at which manifold pressure increases to the point that the ecm's max boost trigger is reached.