isx15cm2350 400hp turnup to 450?

I have a isx15 cm2350 400 horsepower 408k miles 10-speed automatic

When I bought the rig I had it dyno’d and inspected by Cummins and they told me it has great horsepower and excellent blowby for the miles. It doesn’t make the horsepower it once did when it had 0 miles from the factory but it is currently putting about 340 down to the wheels. Tech told me it can easily be turned up from 400 to 450 by some quick and easy ECM modifications and would not need any parts changed, also said transmission is safe for 450

My MPG is quite low and I am thinking it is because of how hard my engine is constantly working to pull 20-40k lbs. My power going up the smallest incline is non-existent.

Is it worth turning up to 450? Will my engine have better power and MPG without sacrificing the longevity?

 

Not even the MPG? 400 is significantly less stronger than 450

 

Cummins said the decrease in power is normal for the engine with 400k
Any stock engine at 400k will not same power it did from factory.
Blowby is under the 1” per 100k miles expectations

Got DPF baked/cleaned recently
Nox sensor replaced recently
7th doser injector + doser sensor replaced recently

Regens fine, not often either
No codes in 7k miles
Mechanically rig is in great condition