Unlikely the turbo actuators, rob beck says they are fine on n54 and doesn't replace them. But in the scenario that they are buggered: Bmminiparts can sort you out. Forge also make n54 turbo actuators.
But If its at the turbo rather than a vacuum problem it's more likely to be a sticky wastegate issue.
Take it to a turbo specialist rather than a BMW indie, I've found they have no idea about twin turbo direct injection petrol engines.
Car: 2010 Jaguar XFR (Cat-back exhaust)
Gone but not forgotten: Highly Modified E90 335i LCI M Sport Manual (circa 480BHP/500lb ft)