I wish mine would get that terrible of mileage empty and towing.
12.6 empty city/little highway 9-10 towing, my truck weighs 8450 empty.
If your going by the instant DIC mileage, that's your first mistake.
It will not even be close to accurate until you have several hundred miles since the last reset, and you still need to hand calculate for mileage.
The DIC is for entertainment purposes only.
My guess is:
Sluggish could be dirty airfilter and fuel filter needs replaced, a few things to check out.
You can NOT delete any emissions crap without proper delete tuning programming.
Anyway to track down the previous owner, maybe thru who you bought it from to find out what tuning you have?
The tuning could very well be EFI, any handheld device any tuner could have been used to load the tune.
EFI is the easiest to install and you do NOT have to put the vehicle back to stock to use the tuner on another vehicle, like most if not all can tuners.
The big concerns for me running high hp for better mileage, is that couple tenths gain really worth issues that come with adding more hp than the drive train has to handle.
A couple of tenths was the only gains I had with bigger hp tunes.