BTW, in case anyone is thinking, "why would I need more than 167 transactions per second", imagine that you have a web site that displays everyone's "last active" time (for chat maybe). If users are performing some activity once every 30 sec, then you will be pretty much be at your limit with about 5000 simultaneous users.

Robert, did you test MySQL with a parallel load? To give it the best chance at maximizing tps, it will need to be able to combine multiple pending transactions in a single write. Also, can you tell if it's updating it's b-trees? (if not, it may not be able to sustain these rates)