Working off a ladder only, I'll get about 350 sq ft a day of 4" reveal clapboards with a 4" brush. Working on the ground or off an Alumapole, I'll get easily 600 sq ft an hour.
None of that includes cutting, just open wall.
I'm pretty fast with a brush, and for something like this I use an old set of Sherwin Williams Wave brushes. They hold a ton of paint, and I load them very very heavily (brush out of the can about half full, and slap the sides after dipping - forces the paint up into the well. If the brush is drippy, slap harder).
Fast is what you want to be if you're brushing exteriors - there is very little time for leveling, so avoiding over brushing and being done quick gets a much much better looking job. That, and working out your application strokes to have the last stroke go into the prior wet edge - like tipping off, but it's an actual stroke, just a little lighter toward the end. The brush marks should be there, but not terribly noticeable, and they should follow the clapboards. I generally use a 3 stroke pattern for a brush load, but on a rough surface it can go to 5 strokes.
No offense to anyone, but a lot of pros go too slow to get a good brush job. Just work out your loading method, see how far it will go and your application pattern, then go as fast as you can and still maintain consistency / control.
The problem with doing brush work like this as T&M is you are going to be working your butte off to do it well, so you better boost your rate. It's better to take a break than slow down - there will be a noticeable difference in the brush marks if your speed is varying.
Old house exteriors around here tend to be brushed, period. The old oil paints dried slow enough you could take your time and they'd still level pretty well. It's a traditional look for a traditional house, but tough to get with current paints.