The V8 doesn't have to work nearly as hard to get it up to speed, and keep it there.
You'll can't compare a V8 to the I6.

As for mileage, keep with the same tricks people use for years:
Clean the injectors
Check the filters
Run synthetic in everything
Tire pressure, Tire pressure, Tire pressure
Make sure the brakes aren't dragging
Don't lay into the skinny so hard
Stay off of the brakes
Improve airflow
Better ignition/tune-up parts
Make sure the alignment is spot-on
Check to see if it's throwing any codes
Lighten it up
See if an A/C delete is avail (about a 15-20hp savings)

And the fancy plugs are just that, fancy. EVERY Chrysler I have ever worked on and owned loves the hell out of Champions. And keep the gap at .035", and NEVER trust that the plugs are pre-gapped out of the box. Example: I could have ran the same plugs in my Olds 350 (528hp) as I do in my Chevy 350 (bone stock), the Chevy got a .035" gap, and the Olds had a .065" gap, exactly factory specs. (If you're wondering, the heat ranges were different cuz of my CR in the Olds...)
The manufacturer will tell you if they are and not to mess with it, and honestly I can only remember about 2 kinds of plugs that you don't gap. But don't start messing around with the gap, unless you REALLY know what you're doing...