I've owned many a Ford V6, from a Ranger to a Mustang, and now I own a faster Ford w/ some Steeda work, but I'd always toss in a K&N since my local guy gets them cheap for me. Was there a difference? Yeah, probably a couple HP. Did I ever notice, not in the Ranger. It was more pyschological, I "thought" I noticed I was going faster, ah, silly college me at the time.
Tear out the catalytic converter. You will get an extra 15hp with a straight pipe but it will be noticable louder.
With the Mustang the above procedure definitely wouldn't net as high as 15hp, at least not all converted, my friend had that done (until inspection at least) and didn't get that much out of it in his Mustang. Not sure how much you could net with the Ranger, I never touched the Ranger that much.
That being said, it is usually affordable and a good idea to get a good exhaust system. I upgraded my Ranger to a K&N and a new exhaust system, that's all, and noticed a lot of difference. As for any kind of gas mileage increases for any of this, I drive my cars like I stole them, so I never get good gas mileage, ever.
Still, after this long drawn-out response, I say get it, it's much better than the stock air filter, even if you won't notice much performance increase. And you can clean K&N's rather than get new air filters when you get tuned up, and at least at my garage that saves me money in the long run over owning my car for five years. K&N is much better quality than the Ford Motorcraft paper filter crap.