So I just completed a full run with the witness config, verified the result, and it came in at 47 minutes (to benchmark block 840k), probably because I took out the 5 snapshots from default config. I also recomputed the theoretical limit based on block download alone (no other messages up or down) of [609,888,208,331] bytes, on my (measured) 2.3Gbps Internet to the top block [866,881]. It comes out to 36 minutes, where that block took 52 minutes. So we're still only 68% efficient.
(edited)