A 2 x 4 interior wall (I assume there is no option here to make it 2 x 6) will allow a rectangular duct size of 3.5 x 14 inches.

An 8-inch round duct has a sectional area of 50 square inches. 3.5 x 14 has a sectional area of 49 square inches. Close enough that investigation of getting required parts made and fitted is justified. Detailed analysis may help fine tune the required blower flow rate versus pressure loss requirement.

If the hood and blower are better served by a 10-inch duct, then a 2 x 4 cavity rectangular duct is not recommended.

The other option is to make a hood and its exposed chimney a focal point as you note. "Better" is a matter of aesthetics vs. functionality. Functionally, round is "better" than rectangular; straight up is "better" than following a tortuous path in the wall that may be blocked at the ceiling.

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