Well, I mean technically the less heat that is generated in the case, the less electricity it uses.
I dont think this applies however to your system in a noticable way though.
PSU's exhibit this the worst/noticably. More heat evolved, the efficiency drops like a stone. That is why 1000 watt kiddie shit psu's make no sense as the more you use them, the hotter they get, the less wattage they put out, the more they use, so you basically are left with a room heater.
however, I dont think this could apply to a cpu and gfx card. Maybe I am wrong, I dont know
I do know that my Res1 blue boy tower heats my room up considerably on my "gaming rig" pc that is on 24/7
I just can't figure how it would make less heat. Hm. Well, when you pull off the cpu and gfx card heat from the case, the PSU stays very cold running. that would give you like 20 watts benefit?