Turned up detroit feels sluggish

Hey guys I hope I can get some help here
.
Here are the specs

DDE4 12.7 in a 200 Volvo 770
Fleet air filter
Borg warner 171701 turbo
Full tilt ceramic coated manifold
Straight piped
Rebel127 20% tune.

Its been 6months already and I just realized there has to be something wrong with it.
The truck just drives like a dog, its very laggy unless I keep the rpms between 1600-1800rpm

If Im climbing a hill, it will make 34psi at 1700rpm and the boost quickly drops off, by 1500rpm it makes 28psi, 1400 its at 25psi. I dont think this is right and I'll explain why. It doesnt pull the hills any better than when it was stock.

Yesterday i noticed the truck was pulling like a raped ape for the first time ever.
I was climbing a hill in ohio on the i77, I look at the boost gauge and its making 34psi at 1300rpm and the egt's are at 750-800!!
Usually i dont let it get to 1300 as its got no balls there and egt's get to 900+ and the boost is at like 22psi.

So I got excited and did some testing on some of the decent hills. At 1550rpm it made 36psi, and 37psi at 1700.
I did not have to drop a single gear when I usually have to. It felt like I could make full boost at the tip of my toe

I also noticed on the cluster where it reads the mpg, my instant mpg would hit 2.5 in 13th gear when it's always hit 3.0-2.8 Depending how it feels.

An hour goes by its back to how it always runs, builds boost slow, very laggy id say it feels worse than a stock 500.

There are no codes and the engine light never comes on. I have no idea where to start. Could it be a weak fuel pump? If it could, how was it making all that power out of nowhere and then back to a dog?
What about the boost sensor, could that have anything to do with it?

Sorry for the long post, I hope someone can shed some light. Btw I did try to contact Leon but he's been busy lately.

 

I just uploaded 2 short videos on youtube.
Here they are





Can Anyone with similar mods chime in?