Yes I did to 27c. I've heard conflicting stories though. Some say it gives the parasite an advantage in that it speeds up the reproduction process.
PS : My powder brown is fine again - I dont think I have a white spot problem - Its to off and on to be true..
Saltwater ich encysts and becomes dormant when the temperature increases which could be why your tang got it after being QT'd. The misconception comes from freshwater where one will increase the temp to speed up the lifecycle but these are two different sicknesses