Yes suction is important but I honestly think that suction is the 3rd most important part that a vacuum should have. Now the reason why I don't favor suction as more important is because the more suction that I have on a vacuum, the more it would likely be "sealed" onto the carpet, rugs, sofas, etc. I used to work at a carwash where they have commercial Spencer central vacuums, they have the best suction out of all the other vacuums that I've used. I have seen wherever I would have the vacuum sealed onto the car carpets or rugs, it would have a hard time picking up all the sand, coins or other debris that the Spencer should easily pick up unless if I added some airflow. So if I had a very powerful central vacuum that would easily get sealed onto the carpet, not only it might get harder to pick up dirt but it might also get harder for me to push or pull the powerhead which I don't think I would enjoy. Plus I might damage some rugs or leather sofas if I'm not extremely careful. That's just what I overall think.
In regards to Duovac and other Canadian companies, I think there's also no doubt they sell one of the finest units on the market. I just wish that ACV "All Canada Vac" was still selling powerunits here in the states. If I were to get a Duovac, I would get myself one of the Premium models.
MD does make units in both 120 and 240 volts. And yes you're right about the motors. For example, the S900r (The 120 volt Silentmaster version) uses 117939 Lamb motors and the S900r2 (The 240 volt Silentmaster version) uses 117944 Lamb motors.