If it was fine for a while after the rebuild it must be something to do with when the reduced the boost level. It may well be the Actuator.
Does the car rev up to 8000rpm?
Like everyone says, you need someone with a little know how as your current mechnic could have cost you alot of money had you kept boost pressure @ 1.5Bar :o
Don't worry about the mechnics, we all start somewhere :)
Like Mat says, you need some conclusive A/F readings to determine if there is a fueling issue. I would try to get someone to check the mapping on the E-manage. Could well be on another base map for all you know!
Fuelling shouldn't be a problem if the car is mapped correctly. Your well within the limits of needing to upgrade fueling at this stage, over 0.6bar you might want to consider injectors/fpr etc....