... but the async calls should greatly improve performance under high load.
Have you done any benchmarking to see if that is true?
I haven't done benchmarking per-se but it's very easy to test if you have access to miners that can do ~4-5Ghash/s.
Without patch: "Problems communicating with RPC server" (or equiv depending on miner) at least
once every 5 seconds.
With patch: One every now and then due to network hiccups.
Further proof: luke-jr has a similar (though VERY hacky) patch he uses or bitcoind falls over for Eligius, his is a threaded implementation that would need quite a bit of cleanup though.
I'm pretty sure, whether publicly stated or not, this is the case for all of the >10Ghash/s pools.