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.
2008 E90 335i LCI M Sport / Manual / Black / Red Leather / Avon ZZ5 / VMR710
Mods: JB4 G5 ISO / BMS Back End Flash / BMS DCI / AR Design DPs / ETS FMIC / Meth Injection / Spec Stage 2+ Clutch / BMS OCC / Cool Carbon Brake Pads / RB PCV Valve / Forge DVs / M3 Control Arms / Bilstein B4s / Eibach Pro Springs / Eisenmann Race
Wish List: LSD