Just for reference, my vacuum unit uses 4 x 1200w vacuum motors that I have in parallel, wired in 2 x pairs. That way I only run the single pair when testing low valve lifts. With 4 going it's just enough on a hot day to achieve 28in vacuum on my 2.8 v6 heads at peak lift. Something else to be aware of is heat, the vacuum motors usually require airflow over them for cooling. So in vacuum they tend to get hot quite quickly. That's one reason I use a air bypass setup to adjust my test pressure, that way it increases the airflow for the vac motors.
Also my flowbench flows a max of 247cfm@10in.