Vista,
You have a premium spa right? You'll get 5-7 degrees an hour on 220v. Your tap water is probably still atleast 50 degrees. That's only about 8 hours until it gets above 100. For me it's not worth completely draining my hot water heater to save maybe 2 hours. Especially with the price of natural gas going thru the roof!
If you want to speed up the heat up time using just your spa, you can always turn your main pumps on for 10-15 minutes with the cover on, every hour or so. I do this about 2-3 times during the course of my heat up process. Mainly towards the end when I get a little anxious. It'll probably bump your hourly average up about a degree or two if you do it every hour.