There are a couple issues here.
First, if you don't drywall the ceiling and leave it open, you're reducing your isolation into and out of the space.
Second, as has been stated, that's a LOT of full range absorption when you consider you also still have to deal with the front wall and the side reflection points - definitely could be too dead overall.
If you must leave the ceiling open, I'd recommend just doing maybe 2' around the perimeter and a little over your head to deal with height issues.
Generally, using an insulated drywall ceiling and a bit of additional absorption on the ceiling AFTER the drywall is a much preferable solution.
Bryan
This is starting to get complicated... I wish I could afford to have a sound engineer figure this out, but I can't.
It will be a few weeks before I will start the actual work, so I have time to figure out something.
So, It would be better to put the rockwool in the celing, then cover with sheetrock (just one layer 1/2" or 2 layer?) Then build acoustical panels to put on the ceiling later?
I want to do what's best, and if this is better, and still will give the impression sonically, of a much taller ceiling, then I need to plan for that.
I really appreciate you guys letting me know it could be a mistake to leave the ceiling uncovered...I was just going by what I was told by others.
Again, any and all suggestions are welcome!
Rick