Are cold air intakes like AFE a guaranteed way to improve a vehicle's fuel efficiency and horsepower?

Crawling along in traffic, sure...  On the open road at 100kph I doubt they do anything unless the manufacturer has designed a particularly bad intake setup.

On the open road the air under the bonnet is not hot, compared to sitting around town at 20kph. Most cars pull air from outside the engine bay anyway, the KE70 gets it from under the guard. They real advantage will be a less restrictive setup as Toyota would want the intake quiet over being free-flow.  I played around a lot with the intake on The Girls KE70 here, but that was because the twin SUs would have been sucking air from straight above the exhaust manifold.-

 

In theory yes. In reality the difference will be immesearable. 

In theory you should see about a 1% improvement in HP for every 10 degree drop in temperature with you cold air intake system. The problem is that you still heat up the intake charge as the air/fuel passes through the manifold and head, so you do not see the theoretical temperature change in the cylinder. But you should still see some temperate improvements and some improvements from a less restrictive intake system.  davew7 

Spray your fuel in at the top of the system and the fuel will cool everyting, atomise amazing and yield better performance than just cold air will.