Nick and Alex,
Thanks both for the help. I reverted the RAM allocation back to 1536 and tried again. I still got the VRAM/System RAM error, and the campaign mode was still slow. I followed Nick's instructions to purge the combat results, then de-selected it as a mod and reloaded. The VRAM/System RAM error is gone, but the campaign mode is still just as slow.
Tough to describe, but when in campaign mode, it's just like watching slow-motion video. The cursor movement is still rapid (no lag). Hitting 'i' or 'e' brings up the respective menus, and those are normal speed. Moving things in inventory is normal. It's only ship movement that is impacted. The ships (and hyperspace lightning, etc) move very smoothly...just very slow.
With the reverted memory and DCR de-selected, I'm still getting 60 fps and ~40% idle time. I looked at the activity monitor for the Mac OS, and it's indicating that SS is taking 1.13 GB of memory. Total system usage is like 9 GB out of 16 GB total. Nothing on my computer is running slowly.