Is this a matching question? If I were using stainless steel appliances and hood, I would prefer a stainless steel wall covering over a galvanized wall covering. On the other hand, if spangles are your preference, then a galvanized surround would make sense. (I realize that galvanize can also be matte by spraying zinc dust onto the galvanized metal when it comes out of the liquid zinc hot-dip coating process.) Is this in a climate of scarce rain? Galvanize will eventually sacrifice itself to depletion and then rust will form. Maybe the question was much different than my interpretation above. If the question is hood side overhang or hood front to back depth or hood flow rate then all of the relevant topics on this forum should be searched. Then adjustment made for expected effects of cross winds and whether capture in cross winds is a requirement. kas

I see no reason to have to go with Miele venting system and unless your kitchen is vet ry traditional I would stay with a simple stinless hood and no wood surround that gets greasy and ugly very often

Usually, a cooktop or range on an island is not good design; are you sure it's your only option? In reality, you only spend 10% of your time at a cooktop or range (actually watching food cook, adding ingredients, stirring). You spend far more time prepping than cooking -- 70% or more prepping! If the island is your preferred work location, it makes much more sense to put your Prep Zone in the island, not the Cooking Zone. What does the rest of your layout look like? FYI...downdrafts are not very good at exhausting effluents from your home (FOGSS -- Fumes, Odors, Grease, Smoke, Steam) They have to work against gravity b/c FOGSS rise and expand, they don't drop and contract, which is what you would need for a downdraft to work well.

Thanks for the response. I would think the CFM has just as much to do with the motor / blower as it does with the diameter of the duct. It's not like you can't push a larger volume of air though a smaller opening given the right amount of pressure. Anyways, here is the 900 CFM insert we are considering with recommended 6" ducting: https://www.prolinerangehoods.com/46-proline-pljl-wall-range-hood-insert-15-deep