Well you dont get a tune for mileage. you get a tune because gm factory tunes suck and you get performance. the Mileage is a benefit.
if they have switched over to winter blend gas then you can kiss at least 1mpg away.
my 6.0 gets 13.5 in the city this past summer, now she's pulling high 11's to low 12's. plus its getting colder.
id get the tune just for the heck of it. id see about putting mufflers back on. could be hurting you some. or run the crap out of it and get a saturn for a DD 30+mpg. thats what i did