The size of the recovery tank and hoses is important, as the vac' motors have to remove the air to reach optimum lift,
if your cleaning stroke is for example 3 seconds, and the volume of tank and hose takes 2 seconds as a rhetorical figure,
then only 1 third of the cleaning stroke, is at optimum lift. This is why each machine has an optimum hose length, and each extension doesn't reduce lift or CFMs, but increases the time taken to evacuate the tank and hose. This gives the impression of reduced performance, but the same performance is there, it's just not fully attained because you've finished the cleaning stroke long before peak lift is reached.
This is why machines run slightly better when the recovery tank is nearly full. And cleaning with longer hose runs, is slower, as you have to allow for the extra time to attain peak lift.
As for motors not being in series, they do wear out quicker, and Amatek advise you do not use 2 x heavy duty motors in series, this is why Ashbys fit one heavy duty pulling through one standard motor, as when they used 2 x heavy duty they suffered a lot of early failures.