I could see it giving a slight increase simply by increasing the volume of air between the TB and the intake valves. But I don't see it being a noticable difference. I would not spend 50$ on one unless I had a larger throttle body to match. And I have seen some fairly "home-made" stuff at the race track in my life. My best one that I would say 1/2 way worked, was my buddy (engnieering student) made a small aluminum lid & intake for his LS1 Camaro, then made a second casing about 1 inch around that with a pressure release valve, filled it with broken dry ice... so essiently held the ice around the intake. His results were "okay at best", and 1 block of dry ice is 10$ and it last about 2 runs on the track under a 195 degree hood... In the end I think if anything it could have build condensation, and adventually did more harm to engine internals that good....
My two cents really tho, is I worked on a project for a venture captilist firm for a company called Optimal Solutions, and they are a fluid engineering firm, that produced a software that can maximize engineering effiency on many products, including race cars. They work with Aston Martin, Honda racing, and other HUGE names. And I saw about 20 ways to optimize intake manifolds, and exhaust manifolds based on air flow and heat structure. But no one ever brough up a TB spacer. The problems in an engine is getting air to make a 90 degree turn, not volume coming in.
I also think if you get a tb spacer, and it somehow does actually help, I think it would take well over a year to see a gain in gas milage vs. the cost the of the product.
Stick with the basics, open up the intake with a cold air of some-sort or just a k&n filter and cut the box opening, and a professionally bored and polished Trottle spacer, possibly headers and opening up the exhaust. But even at that, you have to worry about proper amount of back pressure, if you open theexhaust too much, then you lose more than you gain. Or at least that is how it works on a v8. I am new to jeeps, but have an idea of how to increase power, and getting the most power for your $$. I would say drop a k&n flat filter before you do a TB spacer.