whats wrong with my zafira 2.0 DTI (2001)?

ive lost alot of power on my zafira ive had a new turbo fitted , a new mass air meter fitted , ive had it to 2 independent diesel shops and no faults show up on the diagnostic although one of them thinks it may be the EGR valve , it dosent blow smoke at all and it runs and sounds as quiet as a mouse , help!