I thought most intercoolers were aluminum, in which case you wouldn't have to worry about rust. However, if the water is really hard you could certainly get calcium build-up, plugging the small fins of the intercooler. That's why I suggested maybe using distilled water. If you decide to go the meth/water route, let me know I have a meth lab in my kitchen and can get you stuff real cheap.
You couldn't really use a water spray on a track day. Spraying water on the track is a big no-no. That's one of the reasons why people use a nitrous spray - it's more track friendly. Would it make a difference? I'm not sure. On a dry, hot day with little humidity you would gain a bit from the evaporative cooling effect of the water mist. Methanol would amplify this effect. On a cooler day or a very humid day I don't know if you would notice any difference. Gary, you just need to get out of boggy, soggy Louisiana and move to California man!
The other issue is trying to understand how air flows over your intercooler with the mounting you have. Does air flow from the top to the bottom or bottom to top? You want the misting spray right before the air flow going into the intercooler. If it flows from bottom to top, then you're out of luck as you don't want spray nozzles hanging underneath your car, close to the ground. They'd get yanked off by rough roads in no time...