You'll probably find best results in the 120-140 degree F range. In the few expirements I did, I found it difficult to nail down the benefits because of the constantly changing variables while driving on a road. I think you'd need to be on a dyno with a constant load and then vary they intake temp to be able to detect the changes in fuel consumption. Never the less, I still run one because the theory makes sence....I think
Oh yea, almost forgot about the Hot Ram Air idea. I've thought about and played around with the idea, but found it difficult to get enough heat transfer to significantly heat the air as it moves through the radiator, or an additional heater core like I tried. You gotta get really hot to get a significant heat transfer going...like off the headers.
Try the HAI and check back in!