Actually a warmer intake charge will be more efficient by atomizing the fuel better but it won't make as much power. The cooler charge doesn't atomize as well but since cool air is denser it makes more power.
Well, you could just heat the fuel and spray it into a cold air charge. Then cool off the mixture even more downstream of the carb.
edit: I did a google search for this and came up with an engineering forum thread that talked about this. Seems many of the engineering students on there couldn't come up with a definitive "yes" or "no" on its effectiveness.