I have a 1998 Expedition too. I can see how you would say it lacks some power.
I think it is probably one of the weakest of all the Expeditions - I believe the next year they increased the power, then again later on, then again. My father's 2006 5.4 has truly noticible power - its impressive.
But with the 98, I too have tried a few things. I reprogrammed the computer using an online purchase for doing that for the Expedition and put in a K&N air filter for the heck of it. Overall now its not too bad, but it will downshift when climbing hills on a highway at 65 mph or so...unlike my SHO or my fathers Expy which can stay in OD while highway-hill climbing.
Still the 98 Expy is the one I have and it performs well enough to keep me having it be the vehicle of choice for weekends and any trip!