sorry but you wont save 1/2 on your electric bill. not even 15%. it will be slightly more efficient but not enough to notice. if I come over & pee in your pool,I made it deeper. right? thats the type of difference you are looking at.
instead of running 2 ballasts for your test, try running 1 for a week at 110 volt, then switch it to 220 for a week. I bet I could find 2 identical ballasts that have a 5% difference right out of the box.