Ceiling may have 5" in the center, but that tapers down to near zero at the edges.
The roof trusses have a taper, and typically will give you 5" in the center but they taper down to less than 2" at the ends. Your talking 8ft long trusses and only about 2 ft near the center you might be able to stuff some insulation in.
The there is an issue of lack of air space above the insulation, pack the insulation against the roof and you no longer have valuable air gap between the roof and insulation. Removing that gap will allow easier thermal transfer of the heat from the roof into the insulation and then to your interior space. That air gap above is intentional for a reason.
Your going to go through a lot of hassle for maybe R1-R2 increase average on the entire roof. Very little gain for a lot of hard work and hassle, you will never notice R1 or R2 increase.
If you really want more roof insulation, go big or go home is the way to go, create a 2x4 false ceiling not touching the existing ceiling then fill the false ceiling with 3" worth of poly Iso, leave an 1/4" air gap between each layer and the existing ceiling.. Each 1" thick poly Iso gives you R6 so you get R18 plus each air gap between the boards will give additional couple of R values..
Say you have three air gaps you get an additional R6 out of 1/4" air gaps giving you for a total of R24 increase.
