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The thing about factory cars are they are full of compromises to make it work for everyone. Adding a spacer is not going to hurt anything except maybe 5000+ rpm top end power. A longer intake length will typically give you more power at lower rpms. To properly see what size intake you need is a lot of math and it is easier using a program like Engine Analyzer Pro to change the numbers and see how the engine responds.
I will be adding a spacer to my Astro this summer to improve it's towing ability. I doubt I will see a mileage increase with this van, but it is pretty well known adding a spacer to a tbi engine gives it better response at lower rpms while sacrificing top end power. Perfect for a vehicle primarily used for towing and hauling stuff.
I have the spacers made up for my Metro but that car is sitting in my garage waiting for warmer weather so I can swap heads and put the spacer on. I am not sure if I will be able to do an A-B-A type test with it but I will try and come up with something to see how it changes things. My main goal with it is to be able to shift gears at a lower rpm. If it can drop my shift points by 200 rpm without lugging the engine then it will be a good mod I think even if it doesn't directly improve mileage.
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