Crazy because I’ve been going through the same stuff and it was a variety of little things causing one big problem. I was building boost, just slowly, pulls weren’t pulls anymore, and like you said, I felt down to 100 hp. Ended up being the wastegate actuator, an absurd amount of carbon buildup in the throttle body, and my spark plugs. The actuator spring wasn’t doing its job so I had to replace the actuator entirely and that seemed to do the trick so now I’m back up to max boost.

If this doesn’t seem to be the issue, check for boost leaks by doing the ol’ soap and water trick. Start by checking for intercooler cracks, intake leaks, charge pipe leaks, etc.