Yes, this is what I thought about, too. It might be good idea to look at the memos instead of the voter list of a post. Too make things simpler, I'll try to take the SBD or STEEM send alongside a memo as a proxy for the bought SBD on the vote. This might not be a hundred 100% accurate, but maybe good enough.
Currently the bot runs on a rather small VPS with 2 virtual cores and 8 GB RAM. The latter is the limiting factor (I'm always using about 60-80% of it, and I already need some quirky Python hacks to reduce the memory footprint). Maybe if the bot earns enough SBD on a regular basis, I'll move it to a bigger machine.
I completely understand the memory problem, I run a bot doing an absolute fraction of yours and it takes a good 30% though I haven't looked much into speeding it up because it comments regularly and since there is a limit of 1 comment/reply every so often (20 seconds) it just, I need it to be slow.
I do really like the idea (I thought I had upvoted before, guess not) and will definitely look forward to this to see its progression.