X15 Bad fuel economy and sluggish power

[technyne66]

Hello, I mostly looking for validation before I go and spend $5-6000 for a new one box, but my problem I'm having with my 605hp X15 EPA18 in my 2020 W990 is overall having bad fuel economy 4.5mpg vs 6.8mpg and partial lack of power.
I did hit regen limit at 250,000 miles. I did 6 service regens in a row and this restored quite a bit of power. It was slugging way harder then normal. I'm talking doing 35mph where I normally don't fall under 45mph. However I still can't get to that 45mph, it's more of a fall to 42mph. I have insite and a {garbage-can tuning box}. Both installed and uninstalled, there is basically no no difference in pulling power. My buddy has a DD16 600hp freightliner that I use to pull hard and now I get dusted a mile.
With insite, all parameter seem to be normal with the exception of what I believe to be to high of EGT when climbing. I have quite a few hills here in mid Washington to the west coast. EGTs seem to get up to 900f while climbing but there's no codes, no thermal management kicking on, back pressure is 55psi, boost is 32psi. Everything looks exactly the same as day one of ownership and throughout to this incident. Its had a valve adjustment as well to rule that out but I'm still getting no where.