Some highlights of your problem space's requirements:

• The hood needs to overlap all of the cooking zones by enough to capture at least a 10-degree plume expansion angle (from vertical)
• Baffles are needed to handle that much grease
• Flow rate needs to be at least 90 CFM/sq. ft. of hood entry aperture
• A transition to rectangular duct may be called for to use as much of the joist space as necessary to minimize the pressure loss you have to deal with
• Use an outdoor blower that is rated at at least 1.5X the product of the hood entry area (sq.ft.) and the 90 CFM/sq.ft. value above
• Provide necessary make-up air

Round transition to square will allow for greater volume.

How many 90°, 45° or similar bends will be required in your duct. Besides the overall length and size these will add additional static pressure.

A square duct sounds like a good plan. Using proper vanes in the transition should help reduce static pressure due to venturi pressure.

A larger aperture and containment area will improve effectiveness with whatever airflow you can get with your duct limitations.

More: https://www.gardenweb.com/discussions/5161173/hood-faq