When you read through the specs. for your Viking rangetop (look for them on the Viking website page that shows your unit) you will find out their suggested CFM rating. The rule fo thumb is 1 CFM per 100 BTUs.
For instance, I thoroughly read the specs. for my range twice just to get a handle on this. My range is 102,000 BTUs total which would lead me to at least 1020 CFM. I'm looking into a hood that is rated for 1100 or 1200 CFM.

The power of the blower is directly related to the duct run, if you are on an outside wall going straight out then 600 cfm is more than enough, what complicates this is the length, size and amount of turns in your duct run and what quality installer you use

If you get a system that has a very high capacity, you run into the problem of having to put in a make-up air system as well, which drives up the cost by as much as 50%. the reason is that a really high capacity system will create such a strong draft that you need to bring in fresh air from the outside to compensate. some say you can open a window to deal with that problem, but do you really want to, especially if it's 20 degrees outside.

It ultimately depends on how much heat you expect to generate in your cooking? Do you cooking for a crowd several times a week, and do you have most or all burners going simultaneously, and do you do really authentic stir-frying at super high heat? If yes, then maybe you will need a restaurant-style system with make-up air.

Unless you have a grill which will generate a lot of smoke, your KD is right, 600 CFM is fine. Of course, sales people want to sell you more.