X-15 help

[Flap]

I need some advice. I have a 389 Peterbilt with the X15 Cummins 605 hp, recently it has been demandated. So I am a company driver but my company lets me take care of the truck normally how I see fit. Started having problems with the DPF, that whole story, so me and my company decided to bring it to some shop that they supposedly know real well and does great work. So they did the DPF delete and they also removed the EGR and EGR cooler. And whenever I went and picked it up and seen that they had removed all that stuff, that was the red flag for me, I knew these guys didn't know what the hell they were doing. I have since put the cooler and all back on my truck myself. But now I'm still left with a bad delete, the truck is much weaker than it ever has been it feels like it's only got about 60% power. So we go back over there and they say that it's because it doesn't have any back pressure that it doesn't have power anymore, and the only thing you can do is put a horsepower upgrade in it, And so obviously we did do it, not knowing any better. And it worked a little bit it's still only at about 70% of what the truck originally was at. It's running good the check engine light isn't on and it's only going up to about 30 pounds of boost, I just want to know where did the power go and where do I go from here. I even talked to the guy at my company and said maybe we should put everything back like it was and he said no that's out of the question, we done spent four grand. So anyway he wants to bring it back over there again and tell them that truck has no power, and I keep telling him we should not go back over there cuz those guys are idiots. And I think he might would listen to me if I knew exactly what to tell him. And that's where I am right now, any suggestions. Please don't be too rough on me for my ignorance, thanks