Siga Majrex T&G Ceiling – Drywall or Not? – GreenBuildingAdvisor

I will soon be insulating and finishing an interior scissor truss ceiling in climate zone 7 with a vented attic above.  The bottom truss chords will be insulated with rockwool batts and cellulose will be blown in above the batts.  Siga Majrex will be used as an air barrrier and is properly draped over all partition walls.  The finished ceiling will be tongue & groove pine and it has been my intention to place 5/8″ drywall between the pine and Majrex for added fire safety and modest sound reduction; however, I am now tempted to install strapping to create a service cavity below the Majrex to hold the insulation prior to tongue and groove installation, reduce penetrations in the Majrex, and add the benefits of a service cavity.  I will need two layers of perpendicular 1x strapping so that the tongue & groove can be installed perpendicular to the trusses, as desired.  It feels like a lot of layering (and weight?) if I have Majrex, strapping, cross-strapping, drywall, and tongue & groove.

So, my question is whether it is acceptable, or a mistake, to cut out the drywall.  Does it provide meaningful fire protection or sound attenuation worth keeping it in the assembly?  What would you all do in this situation?