2013 Cascadia/DD13 Power Issues

I recently purchased a 2013 Cascadia with a recently deleted DD13 and an Eaton 10 speed with 505,000 miles. The previous owner did the delete. I have no record of when they did it and for what reason, as they were a corporate fleet that keeps track of all of these things. They said their reason for selling was because it was deleted. I suspect that they were having issues with it and one of their mechanics went rogue and just pulled the trigger without any authorization. That just what I'm kind of reading between the lines of the last few work orders/service records that the previous owner provided to me...

Anyway... I just had this thing through the shop. Full service, new brakes, inspections, check engine light/fault codes fixed... the whole nine yards. It did seem a little lethargic when I bought it but bobtailing about 15 miles from my house to the shop a few weeks ago, it just fell on it's face in the road gears 8th, 9th, and 10th. No power and delayed throttle response. You'd mash it and in about 2 or 4 seconds, the turbo would decide to spool and you'd kind of get some reaction. My guy went through it all hoping that maybe we'd get lucky with some plugged fuel filters or something. Not so much...

I took it for a 6 hour round trip to pick up a trailer that I bought (a side dump). Bobtailed 3 ours there and the power and throttle response seemed a *little* better but it was hard to tell. I noticed that there's visible black smoke now and it's not consistent. It cuts in and out and you can hear the turbo hunting just on a flat straight away. Got the trailer and it went about as well as I thought it would except the black smoke is thicker. Once you get it to road speed, the smoke is present the entire time and you still get the momentary clouds of thick black and you still hear the turbo hunting... this is with the cruise control on and on flat straight roads...

So... What do you guys think it most likely is? I think the turbo at least needs to be adjusted. I sure hope that it doesn't need replaced. The previous owner was replacing injector harnesses as things like that but I don't know what would have made them lean more toward the injectors rather than the turbo, especially if you can hear it hunting the way it is.