I read a study about hot roofs and cellulose and it depends on what your climate is. If you live in Indiana and want to put cellulose in to make a hot roof, you will end up with moisture on the roof deck and in the cellulose which will rot your roof deck and cause mold. They suggested 2 or 3" closed cell spray foam then do cellulose.
I have 2x8 rafters with r19 cathedral ceiling and r11 on the knee walls. I have thought about replacing the cathedral ceiling insulation with foil faced polyiso leaving 1" vent against the roof deck and stacking up the polyiso to give me over r40 but it is hard to justify the cost and effort of tearing down all that drywall.