I have a Spike 15 gal kettle and have had no problems bringing 12 gallons to a boil with the Avantco IC 3500 unit.. I didn't bother timing it but it wasn't excessive compared to my propane burner, sure was nice to brew in our heated garage without having to worry about opening doors and or windows. I'm sure if you did insulate the sides of your kettle it would be that much quicker to heat and more efficient. I own a major appliance store and we have sold induction ranges and cooktops for years, we have yet to have a customer come back and say they wish they would have purchased something else. When my wife cooks something messing on our induction range she will put down newspaper over the cooking surface, then set a pan over the newspaper, when shes done she just tosses the newspaper into the recycle bin.
Pete