I will be installing a best by broan vent hood (K42) in my kitchen. The instalation guide calls for a 8 inch round duct throughout with the least amount of transitions and elbows. The guy that is installing the duct work used a 8 inch round on both ends but then transitions them to an oval, eliptical duct that is 10 inches long by 3 inches wide, looks like the same duct that he used for some AC ducts in other places of the house, kind of flat looking He says the this will carry the same amount of CFM as the 8 inch round. Is this true, or is he just looking for and easy way out.

Please Help