Muffler replacement time...

Not sure there is much to gain mpg wise, I would replace with oem parts, the restriction in the exhaust is already done by your dpf and def systems. I think you are doing good at 450,000 miles considering how hot the exhaust is on these new emissions trucks. I had to replace a flex pipe that was leaking at only about 145,000 miles. Pete sure is proud of that short pipe it was around 300-400 bucks iirc.

 

I would replace it with a hi-flow muffler, Donaldson makes a few different models.

 

We put the Donaldson m1000463 on my dads weed burner t600. It took some coercion with a hammer and cutoff wheel (10 yrs old, over a million miles), but it came off just fine. It has superior flow according to the charts online to the stock muffler and gained him a couple tenths mpg. We picked it up from the truckpro warehouse near our house for $120 I think. And it is as quiet as the stock, maybe even quieter since it doesn't have rusted holes at the mounting brackets like the original did.

 

450K and you need new mufflers? Wow.
No truck I have been in has really needed it that soon. Actually, none of the trucks I have had has needed mufflers. And I idle a lot!

I have no idea about truck mufflers, my only experience is with cars.
They might sound better and may give a slight performance increase, but the overall cost usually isn't worth it.

 

The only reason I'd change a muffler is because its rusted out. If the muffler is after the aftertreatment system there should be no soot in it anyways.

 

Get rid of the DEF system, that's the only way you can improve it.