Consider this;
Drywall comes in 4 thicknesses: 1/4", 1/2", 3/8", 5/8"
If I were you, I would have 1/4" drywall screwed over the T&G. Dependant of the ceiling height, you might not even notice the lack of a substantial " gap" between the wood trim and drywall. Since you have a great backing...