Phil, just wait for EFI. I bet you'll be surprised at the increase.. Till then, keep clean filters, wax the paint and coast more often.
I was getting low to mid 16's with my 05 when I first bought it, deleted the
cat and put a 30hp smarty tune on it. That bumped it a solid 1 mpg. I can pretty much bank on 17.3 mpg now. That's roughly 60 miles a day at 70% highway, keeping the speed in the 60-65mph range, running windows 3-4'' down and no a/c. I have 3.73's and 285's.