I was taught by the brother of Mike Palmer (who started hydramaster) They worked hand in hand for many years before he left and started his own cleaning company. And he would always tell me how they tested cleaning pressures to figure out what worked best. They would take a tank of water and partially fill it with sand, then they would take a spray jet and hit the sand with it to see what pressure moved the largest amount of sand. That’s how they came to the conclusion that 350-450 was the optimal pressure. I could be completely wrong but it’s information that came from some of the original people in the industry. Counts for something in my book and it hasn’t let me down yet
I agree with that wholeheartedly. There are a lot of obstacles that affect the performance of the water at the floor when you dive into it. All setups clean, that's not debatable, but the level of cleaning and the speed at which it takes place can be improved if you want to improve it.
A commercial cleaner and a residential cleaner have different soil loads on different carpets, different prices per ft² and clean vastly different amounts of carpet per job and both can be very profitable with the right setup. Those setups are very different.
Robs monster fuel fired TM for example with the huge pump, 3/8" solution line and high lift blower arrangement is going to blow out a 20 hp TM in a commercial environment, but only be around 25% faster in residential because of the details he addressed in the build and he doesn't have to operate at extreme high pressures to do it. The Devil is in the details.