These range hoods may have a blower rated for 800 CFM in free air (zero static pressure), but they surely do not flow that much when baffles or filters are in place and ducts connected and, perhaps, inadequate make-up air is supplied. A more likely value would fall into the range of 500 to 600 CFM.

If you knew the actual flow rate that could be achieved considering at least the baffle/filter restriction, it would be helpful in evaluating whether the 1000 ft/min to 2000 ft/min rule for minimizing grease accumulation in ducts was better met with a 6-inch or 8-inch duct. It is certain that the 8-inch duct would have less restriction, but the duct, unless very long, would likely not dominate the pressure losses present, so expanding to 8-inch duct may not raise actual CFM by a lot. It would, however, save costs if a higher flow rate hood were to be installed later. Just avoid operating a larger duct at much under a 1000 ft/min duct velocity (at maximum blower power).

Very interesting. The hood is on the outside wall, so only a short 90 degree turn will be necessary. So I'll stick with the 6" I guess. Thanks!