In looking at this, I am thinking that the problem may not really be rendering-related after all, even though it obviously appears like that is the case. I think that what is happening is kind of a race condition of sorts, caused by us using FixedUpdate rather than Update in this game. All our other games except for Tidalis universally use Update, which is a big difference.
FixedUpdate was an easy way for us to handle fixed-set timings, but actually it's something that is problematic on a number of levels in terms of handling the tradeoff between dropped frames and increased CPU load during battles, etc. Bottom line is that I think that I can rewrite this to use our own form of dynamic framerate limiter, and that may resolve this problem entirely. If that doesn't work, there are a few other tricks up our sleeves, as well.
Just looking at how much CPU is being used per frame when the game is paused and not really rendering anything, though, I'm really seeing a super egregious amount (like 12x the amount I feel it should be on my machine, and I think that FixedUpdate is the reason. Even on the freaking main menu, it's ridiculous. And it's not the renderer, it's the CPU thread.
Anyway, I'll have quite a bit of digging to do tomorrow, but I am pretty confident I can resolve that, and we'll see where we are after that. Thanks for your patience!