My c15 needs your help!

ive owned my 03 w9 with an mbn c15 for about a year & a half now.
It has 800k mi. She’s been great till now.
The problems are starting. All at the same time..
The engine was pretty much stock. 475 hp. (No flash YET)
All of a sudden I lose power & have low boost. I was getting about 35 psi. Now I’m only getting 22 psi. The boost builds up very slow so very little power on low rpms. I’ve tried pretty much all the simple tests, no boost leaks from the humps, or from the CAC. Turbo looked fine, normal slack, No broken fins & no oil.
But my mechanic recommended to change turbo anyways. So last week I put a new cat turbo. Like I said, no flash yet.. which I’m glad I didn’t invest in yet because changing the turbo did not fix my problem.
But during this process, my mechanic checked the ecm & he told me my hp is set at 550 hp. Idk how or why. Because I didn’t do it & the previous owner (my uncle) knew nothing about it. It had an original cat turbo & original exhaust manifold. So I’m not sure exactly what tune or rerate? I got decent fuel mileage. I cruise 60-65 mph & I get 6-6.5 mpg so I’m not complaining.

I have no check engine light.
So I’m not really sure what to do next.. of course my mechanic explained to me all the possibilities, but he would have to take a look at everything so hopefully we’re not just going to start changing parts, but find the main cause.

A day after changing the turbo my valve cover started leaking oil.

& also my coolant is running pretty hot. These past days have been in the 70s here in Chicago. My coolant temp will run around 200-210 empty & rise to 230 when loaded. My fan isn’t kicking on by itself anymore. So once I manually turn it on the temp drops back to 200-210. No cooler than that.

& a while back I started having coolant leak out the overflow hose. So I’m assuming my head gasket is bad. So I guess it’s time to open the head.

Any tips or info will be extremely appreciated. I will try to answer questions with the best of my knowledge, since I’m not a mechanic.. thanks!

 

Last week I also changed two radiator hoses so I flushed out the radiator to replace them. But I was thinking about buying a new radiator anyways since I’m starting to noticed a small crack on the side where I see a coolant stain dripping down.
I have not done an oil sample though..
I haven’t really looked into it..

 

I had the fuel pressure checked. I guess I will have to ask my mechanic if he did before & after fuel filter. But the pressure was fine.
My pyro reads 7-800 empty at 60 mph & jumps to 900 loaded. On a small hill it will get to 1000 so I take is easy.

But the overheating recently happened this week. I’ve had power issues for about 3 months now.

 

Well we checked for leaks. There are none. I have no mufflers. I have new air filters. The turbo & wastegate are new but I can try it

 

Update.

My mechanic checked out the truck today. He said I have one bad fuel injector. I believe #4. He said it’s not warming up?
We called kenworth, they quoted me $600 for each injector. So i assume I should change all of them since they are original.
I’m praying this fixes my problem.