Tony,
I drove around with the Flowmaster muffler and the K&N filter for a year before I started doing other mods. Those are two great places to start. If I had it to do all over again, I'd go with the XCal FIRST. That is the BEST mod available for the sport trac. I've said it dozens of times here, EVERYONE SHOULD GET ONE!!!
With the xcal, you get improved performance and/or mileage and your transmission will work CORRECTLY. I don't know what Ford was thinking when they programmed the transmission like they did. It's absolute TRASH from the factory. The xcal will fix that.
You will start to see more performance parts now that they've put the 4.0 in the mustang. It's been said by several on this site that putting the 4.0L engine in the mustang was the best thing that could have happened to sport trac owners. It's going to open up a lot of performance doors for us. I'm pretty excited about that.
Good Luck with your horsepower search.