I would raise it to 30"s at first, your plants going from a flouro to HID is whats causing that, the HID is a very strong light and people say your suppose to phase it in and not go directly to the HID. Make sure you have a dark period in there like do 18/6 if your not already that will help.
I know because I went through the same thing when I got my 400W HID. Once they get use to the new light you should be ok. Its more of intensity shock as opposed to heat stress but if its too close it will definately go into heat stress.
Good luck m8!!!!