Funny you mention that. I had an E-Series bus darken my doorstep about a month ago, brought in by an independent shop with the same complaint. They suspected the turbo needed replacement, thinking the repair would be covered under warranty. It took me all of ten minutes to notice the air filter restrictor gauge was bottomed out. I took the air filter out, and power was instantly restored!!! The air filter was an aftermarket no name brand that said MADE IN CHINA!!! Replaced the air filter and charged RETAIL for the diag (I charge for stupidity). CHECK THE BASICS, CHECK THE BASICS, CHECK THE BASICS.If everything around the turbo checks out then it can only be so many things....have you checked the Y pipe they are famous for cracking, VGT veins stuck ( with the IDS manually control the VGT duty cycle ) CAC Tube hose split, and I know this may sound stupid but have you removed the air filter and tried to run it up...could be as simple as a clogged air filter...don't laugh, I've seen it happen in a dealership where a guy had a F250 for 3 days and couldn't figure out the lack of power problem....and after replacing the turbo, fuel filters, Y pipe and FICM, I asked him if he checked the air filter..he laughed but it wasn't funny when he removed the air filter and it pegged the boost gauge....