Will adding HP help with MPG's?

Here you got the problem. Wrong geared or running too fast. 1700-1800 is waaaaay over the limit. Get it down into the 1300 range and see what happens.

 

I gotta go along with Bavarian, those rpms are way high.
FWIW, in similarly configured units, the n-14 will always get better fuel mileage than the cat. They're all gonna scream about this, and I too like cats, but facts are facts.

 

I have C-15 6nz. I've always run it between 1200 and 1500 rpm. It doesn't pull any better with higher rpm anyway.
My last job running coast to coast, I averaged 6.5 mpg with a van. . 20k out and 43k lbs back. Ran about 60-62... no idling.
slower speeds get better mpg

 

Over the course of maybe a year or more my MPG kept dropping. Got down to around 3.8 when my trailer brakes were not releasing and my wheels were locking up. Turned out my relay valve on the trailer was no good. Now I'm back up to 6+