I know VnutZ from another forum (the guy who wrote that article.) It took him about a month to piece all that together when he was bored in the summer. It boiled down two things:
1) the ECU on the Jeep is too dumb (Speed Density system) to react to any changes in intake systems which has been proven from other jeepers via the over all ineffectiveness of all CAI mods.
2)The design of the ARB snorkel is does not favor easy flow. It is basically a large blow molded air tube bolted to the front fender, then fed to the stock air box using a length of accordion hose. As a result, there is little improvement over the stock system.
The end result was to stop figuring your average MPG and you'll be a happy jeeper
TB spacers are hotly debated as some claim that they have run them and are useless, others claim a noticeable gain.
Headers are in much of the same boat as the exhaust manifold is fairly free flowing from the factory. The primary restriction on the exhaust side of things are the head itself. Due to emissions, the exhaust ports are too small to promote good flow. The only I/E mod which has some confirmation of efficiency improvement is the replacement of the muffler.
For the Tundra, I think Toyo uses MAF for all their engine management, so long as you kept your foot out of it you should see some gains opening up the intake. IMO I'd learn all that there is about your engine so you can get the best idea of how something could improve its efficiency. Cowboyjack do you have the V8 or the 6?