I am going to put 5/8" (or another 1/2" with GG, haven't decided yet) for HT. My concern is a soffit outside HT. I have separate furnaces for 1st and 2nd floor. Their ducts run together cross the house and the width is about 10 1/2'. I used 20 gage 1 5/8" metal studs to make the frame for soffit with 3-4 supports to the joists above. So, I have one 10 1/2" long 1 5/8" metal stud with 3-4 supporting attachments to each joist which is anywhere 16"-24" apart. I don't want to add more weight to this soffit frame if I can get away with 1/2".
The anti-sag drywall is interesting. I have to look around.