There is a pretty good chance that it just your vacuum actuator or an issue with your vacuum lines causing your problem. I have 220,000 miles on my original Borg Warner KP 39 turbo and when I used Vagcom to see if the turbo was delivering the required air, it just wasn't doing what it was told to do. I had some issues with a few very small vacuum leaks and one bigger one, but the vacuum actuator definitely needed to be replaced. When I used a hand vacuum pump to apply vacuum to the new actuator, it would go to a position & hold it solid, the old actuator would start going to it fail to position right away. Changing the actuator is a much easier job than replacing the turbo. When I removed the old actuator, the vanes moved freely with very little effort. I found that checking the vacuum lines in a sink with some positive air pressure looking for bubbles was the easiest way to identify leaks. That rigid plastic vacuum line that supplies the brake booster is an item that cracks, I didn't see the crack until after I removed it because it was under the 90 degree bend.