1999 Saab 9-3 2.0/turbo, manual, 159K miles. Barely replaced turbo (with new turbo) at 154K miles. Had sluggish power after new turbo installment. I had informed saab mechanic that the car felt sluggish upon receipt, to which he replied I could pay extra for a “performance” tune up. But, now months later the loss of power problem has worsened. Since I paid dearly for a complete new turbo installation I’d expect the same level performance as prior to the old turbo going out. To be clear: no turbo power upon acceleration, slight whirring noise heard when stepping on gas, turbo pressure gauge stays near end of white zone, barely top of white, but needle doesn’t leave white into yellow zone as it’s suppose to when full gas is given or when revving up rpms to 4800 while switching gears… absolutely no power, nothing. I’m relying on down-gearing for some engine power when going uphill - tis’ that bad. Also, no black smoke or indicator lights. In researching people mention both vacuum hoses or waste gate come up as possible culprits to turbo-related power loss. I’m no mechanic so plz clue me in. thx. 
It sounds like the “turbo” problem prior may not have been the turbo. Or there was something amiss in the installation.
You are correct there are some vacuum hoses, a waste gate(stuck), and intercooler that can all leak and lead to your symptoms.
My bet there is definitely a leak. Maybe take this mechanic for a test drive. Some mechanics are real jerks and take offense when there work goes amiss. Or its possible they may not be a “saab” mechanic however turbo engines are all pretty similar.
There is absolutely wrong with messing up as mechanic. What counts is how you respond and also verify your work prior to handing back to customer. Offering another service without any analysis is bad.
What were the symptoms before the car was taken in and a bad turbo diagnosis was given?