The change to 16KB cache block size was fine, however the impact was almost imperceptible on the usual workloads running on our MD3200. I did keep stats from our backup jobs, however our improvement was marginal (<5%) and possibly statistically indistinguishable from zero?
Since then I have also switched off cache pre-fetch on most of my virtual disks (those not storing backup data). Again, the difference imperceptible, possibly zero.
But regardless, I now have more knowledge about the configuration and therefore more confidence for the future. So thanks again for your work in digging out this information.