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As far as performance mods, there are 2 types. The kind that allows you to burn more fuel, and those that make you waste less of it.
Burning more fuel gives you more power. Often this means NOX or a blower. However, anything but a turbo as a blower is not likely to show you gains in FE. There are also less drastic mods, like flow porting the head (well, maybe that is a little drastic), which make the air flow more easily through the engine. Headers are a very common item to get a few HP, cause they tend to restrict less.
Then you have your actual "efficiency" mods. Many are the same for us as they are for performance cars. Take weight for instance. Go to a drag strip and look at the inside of the cars. Not too much there. Extra mass in the car does nothing but take up precious energy to be accelerated with the rest of the car. Of course, there are also LRR tires, aerodynamics, lightweight oils, proper gear ratios, etc. Some of these are a little fuzzy as to which category they are in, but you get the point.
In theory, removing baffles will reduce the restriction on the intake, but actual testing beats theory any day of the week. I don't claim to know why a modified intake is less powerful and less efficient than stock. It won't even be flowing that much air when you're below 50% throttle and below 2000 RPMs, which means most of the time (I imagine).
And as far as performance mods killing MPG, you are right in some situations. If you have a 350 with a voodoo cam and a big supercharger poking through the hood, it probably has a really bad BSFC chart. But take a well flowing engine that has simply been tuned and streamlined, maybe with a turbo, and it won't necessarily be bad off the bat. I think that when you apply power is more important than how. If you are constantly flooring it and never coasting at all, you are also going to be using your brakes more, which is indicative of the dreaded "car doing work on the brakes" instead of "car doing very little work on the road and wind to stay in motion."
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