Chillers are a great way to go, to lower tank temp., if you can afford the initial cost of the chiller. Larger tank, larger chiller, more expensive. I've always wanted one, but could never find one cheap enough used, or afford a new one.
After giving it quite a bit of thought though, I don't believe there would be too much difference in energy consumption between, running your AC
lower ( I keep mine at 75, tank runs at 78-80) and paying to run a chiller. Chillers use a lot of power. They are basically, like running a window AC
unit, for your tank.
Of course there are a few factors involved as well, like how insulated your home is, and how efficient the chiller is. I'm sure newer chillers are becoming more and more efficient.
In my situation too, I have many other pets to consider (cats, birds, dog), and I'm sure they appreciate the cooler house...people too!, other than just keeping my corals and fish cool.
If I could get one, I probably would because they are a great piece of equipment, but I don't feel I spend much more money on power, than somebody with one, to keep my tank cool...JMO.