The suggestion was to use it as a fire barrier. The pipe is already insulated and the chase will keep it warm. I would think that you don't want the chase temperature to get too high or you start having concerns of pyrolysis. FWIW, a whole lot of chases out there have no insulation at all. My suggestion is that Durock is a better product to use than drywall because it's non-combustible, with no papered sides.