i/o buffer size, 8 second wait for playback?

When I moved away from Final Cut Pro to Premiere, I also transitioned from Soundtrack Pro to Audition. Overall, Audition is far better--multi-core 64-bit, doesn't crash all the time and it has auto-save for when it does.

However, I'm always hitting a barrier in my sound design projects. I'm mostly working with trailers running 30-90 seconds. My final projects have on average 40-80 tracks. When I get about 70-75% completion on the project it becomes incredibly cumbersome. I try pre-rendering all tracks with FX, then disabling the pre-rendering but that doesn't seem to make any noticeable difference. I typically stick with a 48k sample rate and an i/o buffer size of 512 samples. That's totally slick. When the project starts to cause dropouts of some tracks on playback I jump up to 1024 samples, and that slows down iteration and interaction time but usually still workable. When dropouts happen again I bump up to 2048 samples, and then it's like a brick wall. I hit play and watch a beachball for 8 seconds before it starts playing every time. 

Is there a way to get quicker interaction at 2048 or prevent dropouts at 1024? I'm on a 2013 Mac Pro 8-core, 64GB RAM, dual d700 gpus, 1TB PCI-e flash boot drive. I store my source media on my external RAID array which sustains over 800 MB/s read and write. I'm previewing out to my reference monitors via a Blackmagic Ultrastudio 4K. Is there another hardware interface that I should purchase to avoid this brick wall that I keep hitting?