Last night's tweak fest and refactoring yielded really significant results. As I mentioned, it was session capping ad delivery that was bringing me down, and I think I've resolved that issue. On my old cheap hardware I've been able to get an average of 1.5 million ad impressions per day under a simulated load. Disk isn't getting overwhelmed, CPU time is around 75%, and delivery is under 0.1 seconds. I could easily push it harder, probably to 2 million impressions per day.
I wish I could get my hands on some real hardware to see how it would perform!
I think there are a few more areas I can tweak. I'd like to get to 3 million impressions per day on my ghetto box.