Continuing, finally got my laptop here now.....
(And by the way, this is not to say who's wrong and right, just a lot of people have misconceptions about the subject) 
If there was no Air Flow regulation in the system you would be perfectly correct with the CFM notion and it would also surpass 0.5bar, but in this event there is air flow regulation by the Actuator or External Waste Gate.
I maintain 0.5 bar bar boost is still 0.5 bar boost, the Intake and boost pipes have also not changed size which means the capacity of the 10mm pipe has not changed.
a change in turbo size can not change 0.5 bar sissy boost into 0.5 bar super boost, the difference in turbo size only means that the T3/T4 has the ability to create a higher level of boost than the T25 and further more, an increased ability to maintain the boost, making the ability to maintain boost, the only functional factor in a regulated system.
The boost after the turbo is then maintained by the waste gate to remain at no higher level than 0.5 bar boost, the only time this would go beyond 0.5, is if you have a boost spike caused by actuator reaction time and or if the actuator vent is not of a sufficient size to release enough pressure quickly enough to prevent it.
That is why I say 0.5 bar boost is 0.5 bar boost.... So no, it can in actual fact not cause super boosting unless the actuator of what ever type you are using, fails or is over exerted.
As well, even on 5mm pipe vs 10mm pipe, 0.5 is still 0.5, the difference here is the rate of air usage coming out, obviously the piping restricts the air flow rate, but still will not make ANY change to the fact that there is still a regulated 0.5 bar boost. Only difference is that the T25 will have much less lag than the T3/T4, and probably not maintain the 0.5 bar pressure reliably all through the rev range, where as the T3/T4 will have more lag but will be able to maintain a reliable 0.5 bar ALL the way through.
This now is where the CFM comes in, the CFM rate is the turbo's ability to ACHIEVE & MAINTAIN the boost level or regulated boost level, the size of the pipes dictate how much lag you would have, as well as max flow rate between two point (Amount of Restriction between both ends to effectively do their jobs), but this will still not and can not change 0.5 bar boost into 0.5 bar super boost, pressure is absolute, not variable.
An ECU is designed to supply (So much) fuel to 0.5 bar boost, and that is why, if your ECU is mapped to 0.5 bar boost, and remains at 0.5 bar boost because that is what your actuator or waste gate has been set to, it will not effect your AFR, only your response time, in other words your lag.
