The cat is in the muffler,or it is on the Cat engined Freightliner chassis. Change the muffler. That will help a good bit, I heard someone say, not me.
No DEF it should be fine.
However go online to the Cummins engine forums. Lots of people have Cummins engines.
Ask questions of others running tuners.
Be comfortable whith whatever you do.
A tuner will raise power and increase mileage. I would still install a pryometer, in the exhaust next to turbo or wherever a Cat Rep or Mechanic would recommend it.
It will give you the temps, of the the exhaust near turbo and let you know if you should have to back off.
Remember, to always idle, for five minutes, tuner or no, to cool turbo bearings when you stop. imo.
I don't think there is enough oil cooling going on in my mh,and when the opportunity arrives and extra oil cooler is going on it. Checking the oil can burn you.