I think 22 is the sweet spot. Especially as most of the rider's weight on on that tire. The extra width provides peace of mind when riding slowly.

I can't imagine that there are any aero losses going from 19 to 22mm on the rear disc. The ability to rider slower (even if just in warmup) is a benefit. I guess a 22 would be heavier than 19mm, though.

I've used 19mm on a Zipp 900 disc with a not-wide rim bed (circa 2008). Promptly removed it and put on Conti Steher 22 and was very happy...until I got random flats with several of them.