I suspect you are actually correct in your assertion, contrary to the fact that the application seems very similar to one I'm familiar with where several thousand systems are currently being employed successfully. (Ironically mostly using 11 kw motors with 3 in. suction lines)
I agree in respect to keeping things as simple as possible. However if there is a derivable benefit from achieving constant vaccum across a range of various capacity levels, it is worth considering. In a number of countries there's plenty of this going on in the Dairy Industry, (on farm). I...
In the interests of maintaining stable vaccum irrespective of reasonable demand variations, you may want to consider utilising a 4 - 20 mA pressture transducer and variable speed drive system.