Numbers are coming in and it looked at first like this might have been the order of magnitude change I was looking for, but the time benefit was entirely within the smaller blocks range. Looks like the net savings will be about 1.1 hours, putting completion for confirm at 6.5 hours. However the disk I/O went form a consistent 25mb/sec to a consistent 3mb/sec, so it's certainly a big improvement. Will still need to measure the cost in terms of reading out transactions, which I expect will be slightly higher.