If I add an additional layer of gypsum, or increase the thickness, can I achieve a higher fire resistance rating?

According to section II, Subsection 9 of the BXUV Guide Information, Gypsum board thicknesses specified in specific UL designs are the minimums. Additional layers of gypsum or greater thicknesses of gypsum board are permitted in any UL design, provided the fastener length is increased to provide penetration into framing that is equal to or greater than that achieved with the specified gypsum board thickness and fasteners.
There is no corresponding increase in the hourly fire resistance-rating for adding layer(s) or increasing thickness of gypsum. Fire resistance-ratings are determined by the specified construction in the design based on the UL 263 fire test.