Sorry for the late response, I didn't catch this one before.
1) Yes, works when the A/C is on
2) Power gains are dependent on what you are switching from, there's no definite answer. The killer chiller is able to get the I/C quite cold (they state 50-55 degrees cooler), so I would say 20 hp is quite doable.
Hope that helps. Let us know if you have any other questions.