Already have 1/2 drywall on 3 of my walls. Drawbacks of just adding GG + 5/8 on top?

The basement of my house was already finished when I bought it. I want to convert an existing space that's ~17.5 x 21.5 into a theater. Preliminary layout below.

Existing room has 1/2 drywall on three walls (the fourth side is open to the rest of the finished basement) and a drop ceiling. It would obviously be a ton of work and mess to rip down the existing drywall to start from scratch putting up double 5/8 drywall. Any major drawbacks to putting new 5/8 drywall over the existing 1/2 other than an obviously proportional reduction in mass? Can I use GG on already painted drywall?

I do plan to put double 5/8 on the ceiling with whisper clips/hat channel. I'm most interested in sound isolation for the floor above. I do plan to leave a good chunk of the "back" wall open to the rest of the basement. Will the leakage to the floor above be so great through this back wall as to make it pointless to bother with double drywall plus green glue on the rest?

Thanks.

For aesthetic reasons as well as our planned use case, I'd rather sacrifice sound isolation than close the back of the "theater" (I probably don't deserve to use that term with the blasphemy I'm perpetrating). Plus I'll save money on clips, GG, and extra drywall.

I am interested in why you ask about 4 seats as a single row. That configuration is certainly available. With the width of the screen, the outboard seats would be partially outside the edge of the screen - hence the plan to angle them back in.