I have seen this done before in certain situations. You need to check with your local building department to varify it is allowed by your local code. Before you call the building department you need to call the manufacturers technical support.
10" round duct = 400 cfm
3 1/2" X 14" rectangular duct = 150 cfm
Ideally the duct run should be as short as possible with as few turns as possible to reduce the build up of grease. On average most people fail to clean the grease filters on the hood often enough. Ductwork size is based on the friction of the moving air versus the duct itself and the blowers ability to counter this friction. I think the 3 1/2" X 14" duct will be the limiting factor of the system.
Hope this helps.
"Quality is never an accident; it is always the result of intelligent effort. The bitterness of poor quality lingers long after the sweetness of low price is forgotten" --- John Ruskin