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If the engine comes from the factory with a low threshold for detonation (i.e. high compression ratio in most cases) then you should really think twice about WAI. Not only is there a possible large drop in performance, but your efficiency could go down.
Higher intake temperature = higher charge temperature. While this helps the flame speed, it also reduces the heat ratio of the charge and thus reduces the power extracted on the expansion stroke (as a proportion). It also puts more stress on the various engine components from higher pressure, aka more friction.
WAI is a very crude method of improving fuel economy, as it exploits a fundamental "design flaw" of most engines in a way that has negative effects on some aspects of efficiency.
With Valvetronic/etc. WAI is probably a bad idea because down to maybe 40% VE those engines should be running more efficient at less VE, and especially if they have high compression ratio such as the Nissan VQ37VHR engines.
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