This year I'm also toying with getting
http://www.froggysfog.com/ instead of they Chavet brand I used last year. Froggys makes a special formula for low-lying fog.
http://www.froggysfog.com/category/10-fluid.10-fog.30-freezin-outdoor-ground-fog/
I can attest to the Froggys Freezin Fog's effectiveness. It is simply outstanding, with great hang time. I very easily created a London Pea Soup last year, with a Chauvet H-1100 and a Ness (both are 700w elements- I can't recall the model of the Ness because I sprayed it flat black years ago for camo in the dark). There was little need to run my (2) 400w spot foggers, except in the lowest burst duration I could get. So that they would intermittently spurt under pressure out of my hell hole and cauldron. It actually caused a problem with visibility on a major highway more than a hundred yards away.
At the OP: You can either take one of two routes, either get a more powerful machine. Or alternately, get several low power machines. For your purposes with a chiller, the only real option is a more powerful machine. I'd recommend a good quality machine in excess of 700w for the size area (450 sq. ft) that you have to work with. Ofc, larger is better. As more power allows for less reheat cycle time. Also more power means that physical demands on the pumps aren't as high as you don't have to run it as often vs. near continuous duty with a smaller machine. In turn, that nets you a longer lifespan on your equipment.
Overkill is not really possible when speaking in terms of yard haunts. Especially considering mother natures propensity to play tricks of her own with windy nights, that somehow magically coincide with the one night that we need no wind. Which is why I'm armed w/ 2200 watts of foggers. Boy Scouts motto: Be prepared!
Btw: I almost forgot. Froggys is wonderful to deal with directly. I ordered my fog scented last year (Swampy Forrest, I think it was called), and they accidentally sent just the normal formula. I called as soon as I discovered the error (3 days before Halloween!). Not only did I receive a response from the owner of Froggy's (Adam Pogue-at 2 A.M mind you!), he overnighted me the scent with Saturday delivery! That my fiends, is customer service like very few others provide. I vowed then, I'd use no other, ever!