Enclosing the space isn't much in expense or work. That's kind of the beauty of that space you have.
The choice of sticking with the suspended ceiling, plus removal of drywall to insulate, means you have
chosen to not drive down the room's noise floor. You just lost the opportunity to NOT have to adjust the volume
during a movie. With a typical 55 db noise floor, you won't hear the quiestest passages of a movie, and when you
turn up the volume to hear those, then loud events are way too loud.
Also keeping that access to the HVAC, means noise from the theater has a really easy path to spread throughout
the house. So I would be looking for more then HVAC access to make that decision. I have three layers of drywall
with GG, between my HVAC and theater. The difference is dramactic, and there's longer my wife constantly asking
me to turn things down. She's happier, and I'm MUCH happier to watch an action flick at volume, with great dynamics.
Once I see your choice of insulation and drywall removal, along with realistic seating expectations, coupled with
solid gear for that room's volume, that makes me think I'd use the flooring already paid for. And paying careful
attention to sealing the room, and have something closer to a high end dedciated theater, then a simple media room.
Some planning ahead means you could have a projector upgrade, while using the hdtv for now.
Access to HVAC wouldn't be an excuse to miss all this opportunity, at a very affordable price point, for myself. So I will ask
what's beyond the HVAC up front, that actually needs access? If the HVAC joints are sealed and the system is properly
balanced, then you don't need access to HVAC. (All my HVAC is all my HVAC is behind three layers. One layer of
plywood, and two layers' fo 5/8" drywall, with Green Glue.)
Simple and inexpensive DIY'er built backer boxes could plant spot lights and in wall surround speakers in the sound isolation