I would caution on leaving the rafters open. While the extra height is great, the rafters would most likely would not act as diffusers, but rather create some nasty reflections and timing bounces similar to what happens in room corners. (Proper diffusers are a bit more complicated than what most people think.)
I'm currently designing a dedicated listening room in my house which already has a finished <8' ceiling (drywall screwed to the trusses). I'm needing to open up the ceiling to handling power, A/C, lighting, etc. and was looking at these new low profile ceiling grids (link below).
https://www.homedepot.com/p/CEILINGMAX-100-sq-ft-Ceiling-Grid-Kit-White-18200/100521670
...just my 2 cents...
- Jeff