I did the DIY intake, straigt piped the stock exhaust, and still only get 13 highway, 11 day-to-day driving... DRW 4x4 auto work truck, the van is worse, 10 to 11 is it's best, though it never sees anything but intown driving.
I'd take your 16 MPG anyday...
I know a local guy, his F-250 SRW truck is lighter, geared higher, (3.73's) and has a 6sp stick, he can get 20 MPG stock, but my guess is cause he can lug it in a higher gear, not wound out like my auto truck... he can spend more time with the motor at a lower RPM, and thus less fuel is used, better MPG... for max fuel econo, I would want a stick shift...