It's probably too late, but for information (and if you haven't already done so), I suggest posting questions like this on the "Discussions" side of Kitchens. This is the "Gallery" side that's more for finished kitchens and the like. The Discussions side of Kitchens is for on-topic discussions concerning kitchen remodels. You will most likely get many more responses over there. Good luck!

I have your same problem but have no option to enlarge the vent duct due to the size of the space between the wall and ceiling studs. My previous vent hood was a 36" Cavaliere 900 cfm for a 36" Dacor Epicure cooktop. My new range is 36" DCS with 6 burners. Because I wanted a more substantial look, and after much searching and debating, I bought an AirPro 860 cfm (link below) The Cavaliere hood was more than what I needed for the 36" Dacor. I never turned it up past 1 or 2 while cooking. On 6, the highest speed, it was kinda loud but since I never used it on high it wasn't a problem. The AirPro has not been installed yet so I don't know what the actual noise level will be but it's rated at 35 / 0.7 to 67 / 6.5 (Lowest to Highest Speed). I just couldn't see paying thousands of dollars for a vent hood especially since my $400 Cavaliere had worked fine for 5 years. During my hood research one of the retailers did tell me there was a vent outlet adapter available online but because our contractor wasn't thrilled with that idea, I stayed with the 6" hood duct. Hope this helps. Good luck in your search for the right hood. Here is a link that might be useful: AirPro Hood

^^^^ that! Ductless range hoods are usually installed where there is NO WAY to put in real ducted hood. They are definitely inferior for removing the heat, humidity and odors from a kitchen.

GreenDesign's comment, supplemented by weedmeister's comment is the most succinct. I was imagining using a four-sided duct cover and tiling the entire space including behind it. Then no grout to cover interface is needed. Note that the tile grout will not bond well to the stainless steel, and with different thermal coefficients of expansion, will crack there anyway. Some grout colored silicone would be needed. One could tile to the ceiling or not. Unless the duct cover is kept away from the wall, island hood style, a close fit will avoid having to clean behind it. If the tile stops before the ceiling or soffit, then there will be a gap on the wall side that will be difficult to clean without an air gun. To avoid that the three-sided solution with tile installation last can be used.