Whatever you vent is going to be where the vent lets out. Therefore, the moisture and smells are all going out that hole. My house was built without any duct at all (along with some other weirdnesses). I was lucky that I could vent onto a side yard where the services and meters are. It is also the only outdoors path into the back yard. When the hood is on, you walk through a miasma of housebreath. The vent points down, which is the right thing so that you're not hit in the face with it. Not something you want on a patio or other place you're trying to enjoy, nor by a window through which it's going to come right back into the house. I don't know if it can also leave fog or even particulates on the window glass. Hopefully the filters and gravity would take care of the latter, but Kas is the physics expert, not me. Keep in mind that architects often aren't concerned with function. Wright's roofs leak because of poor functional design in his high art design. Meier designed the Getty Center with way too few restrooms, and by the time they figured it out, it would have cost too much to fix, so they finished building then remodeled. Architects are artists first and foremost, and mostly want to do what looks good, which is why they often go for what will minimally work, but looks cool, rather than using maximal function as a constraint and working that into the design (which is what Deusenberg was famous for--engineering first and a gorgeous coach on top of it).

How is the exterior of the duct configured? Can you get an electrical line to it? Alternatively, can you get to the duct in the garage and add an in-line blower. Once your hood is in place and operational, you can check the outside for apparent flow rate (with some house windows open just to reduce any MUA choking). If it seems as weak as we might fear, you could consider replacing the cap with a supplemental blower or going with an inline blower. It would have to be wired back to a relay that the hood blower power energizes, or else the hood blower control would have to be upgraded to handle both motors' total power draw. This assumes that the hood uses a standard induction motor with one of the usual variable motor controls. Effectively, the exterior or in-line blower would be lowering the duct internal pressure, thereby compensating somewhat for the pressure losses in the MUA and hood baffles, duct turbulence, etc. The Broan (I think) scheme of using an air flow sensor to turn on an MUA blower won't work in this case for the supplemental blower because once flow is detected, the controlled secondary motor will keep itself on. You might be able to double the achieved flow with a supplemental blower. This should make up for using a duct of half the sectional area needed, but that will depend on both blowers' fan curves. You will possibly end up with adequate performance at the expense of another blower and perhaps a higher than desirable duct air velocity (over 2000 ft/min). In that case, insulating the duct in the garage (if the garage is unheated) will reduce grease collection in the winter. Before using insulation, check for vibration resonances and if needed cover selected duct sections with sound deadening material, such as the lead-filled [my assumption] sticky-sided plastic (maybe it is asphaltic) sheet used for automotive interior soundproofing. There is also the product here, but I have no experience with it. http://www.acoustiproducts.us/ Making the differential MUA house air pressure positive would also help the flow rate, but now house wall leakage outward will contribute to moving moisture into the walls from inside in the winter, a problem unless the house is ultra well sealed.

Noise was a factor for me, and I did look at ZLINE for my own recent remodel but found the Wolf on clearance so went with it for several reasons including my need not to max out past 400 cfm on a blower due to building code restrictions for ducted ventilation without make-up air, and Wolf was one of the few pro-style hoods that offered a lower cfm motor. I looked up close to 20 different reviews for you from several sources on the 9597 ZLINE model you linked above. People seem generally pleased with it. Two reviewers complained that it was noisy. One reviewer said it was quiet. Several people said the LED lights burned out prematurely. There's not much more info I can find. You may just have to take your chances if you have a certain budget to meet and this one works for that. It will be quieter if you run it on the lower settings.

Broan/NuTone/Best make a large range of ventilation equipments and generally you get what you pay for as far as I can tell. However, why a "high end" appliance dealer wouldn't include some other brands is unclear. At a minimum, I would expect a Wolf dealer to have examples of Wolf hoods. (My hood is a Wolf, made by Independent before Wolf moved production in-house.) Decide how high you want the hood for good site lines and reaching to the back burners, and missing one's head, while keeping in mind that the front burners will be emitting plumes that expand as they rise, so height corresponds to increases in needed overlap -- side to side and front to back. 10 degrees from vertical measured from the flame diameters when pans are on the burners can be used for plume expansion estimation. Sketch it out. Given a height and hence width and depth, calculate the square footage of the entry aperture of the hood. For wall hoods the light bar in the front may qualify as partial or whole entry aperture area. Multiply area by 90 CFM/sq. ft. Multiply by 1.5 if you don't know all the pressure losses. Find a blower that has that result as a rated value. Decide on in-hood, in attic (in line), or on roof. Farther away is generally quieter. If there is room for a silencer (Fantech) between blower and hood then the result will be quieter still. There is a lot of dissension about ease of hood cleaning by type. I can say that my Wolf hood baffle assemblies easily remove and can be cleaned in a dishwasher. (I have ten of these, so it takes two loads.) One still wants to reach up inside and wipe the surfaces. I use Simple Green, but likely there are myriad possible choices. I have never owned a VaH, and was initially repulsed (2007) by the Magic Lung® claims. Others like them. If you have guests sitting where they can see into the hood, a baffle system may be aesthetically superior. Duct should be sized so that full power actual flow rate achieves a velocity in the duct between 1000 and 2000 ft/min for best balance between grease condensation and grease impingement. You can go lower (larger duct) if you pre-heat the duct in cold weather. On CFM focus. Standard (if that actually ever applies) residential hoods are designed to fit into residential kitchens. Commercial hoods are designed to fit into commercial kitchens. At a minimum, the typical difference is two feet of ceiling height. When the commercial hood can be built so that there is tremendous volume below its baffles, there are conditions where the specific flow rate (CFM/sq. ft.) can be reduced below the value I suggested above. This is due to "baffle averaging" resulting from the reservoir volume vs. plume steady state and transient volumetric rates. At the other extreme, residential hoods with little volume below the baffles will allow plume escape (capture failure) if the plumes are not immediately entrained into the baffle flow. And, of course, plumes not initially captured are not contained and removed except as a matter of long term kitchen (and whole house) air replacement. So CFM focus varies. Another difference is that commercial hoods are typically operated at a fixed CFM and the MUA is balanced for this case, whereas residential hoods, along with bathroom fans, etc., operate at variable CFM, and this makes the MUA control more complex.