I'd suggest this mod has the potential to increase FE depending on the vehicle/ usage. The main reason warm air increases FE is it reduces pumping losses., so warm air is less likely to help on throttle-less engines.
You might also find if your car is underpowered, the extra hp means you can get to cruise speed faster. I'd consider adding the spacer and then plumbing in a WAI that introduces warm air at low engine speeds/ low throttle openings.
Performance and economy
