I came across this looking for the same question you are. I asked around and the answer I got was "the default of 256k is the best for 99% of cases, especially with VMware VMFS partitions". The only time we would consider deviating from this is in a situation where we have a very predictable large sequential block workload (not typical with VMware VMFS') and possibly with things like video streaming where the controllers were struggling to write the data quick enough.
I hope this helps others looking for the same info.
R Hinder