4444913664 bytes transferred in 151.939785 secs (29254442 bytes/sec)
Esoteric command yielding fastest transfer rate of .iso from server to fast flash USB stick.
Really poor performance using a simple 'dd' got my attention. Fixing a one character typo (had left the 'r' off 'rdisk1' ) made performance reasonable but still significantly slower than what I thought it could be given the intermediate link rates. A bit of experimenting with parameter values and structure eventually produced the above ornate command, which runs as fast as dd-ing from /dev/zero to the stick.