"It's 30% less duct run, but does that translate directly into 30% increase in hood efficiency?" The short answer is no, assuming "efficiency" meant "flow rate." The longer answer is that if the pressure losses in the ducting and from exiting the roof vent and from entering the hood aperture and from passing the hood filter are known as a function of flow rate, and the fan curve providing flow rate capability as a function of the pressure loss across the fan is available, then you can evaluate actual cfm as a function of how negative you will allow the house pressure to get. In other words, the duct losses are only a portion of the equation. They may dominate in some cases, but even then the fan curve must be known to make an evaluation. In the hope of making this clearer, assume that the abscissa of a graph is flow rate, and the ordinate is pressure drop across the fan. A conceptually idealized fan would be plotted as a straight vertical line at the fan's rated flow rate to some high pressure drop and then follow some monotonic curve to the ordinate where there is no flow rate. Alternatively, if a fan were to lose flow rate by 30% for 30% more pressure loss, the curve would be an inverse function (concave curve). Real fan curves are better than inverse functions, and better than straight-line functions between rated cfm and maximum pressure loss, but not square shaped. Hence, without the curve available, and some estimate of the other pressure losses, flow cannot be determined. (I once found example fan curves somewhere on Broan-NuTone's web site. They should provide some insight. Note that the curves will vary in shape with fan design.) kas

Is your appliance store recommending anyone? A local appliance store that provides good service should be willing to recommend someone who has done good work for the store's customers. Our appliance store recommended an appliance installer when we needed a modification to our hood and its venting. When the roof cap needed to be changed, the installer felt a roofer should do that job given our roofing type and what needed to be done so he recommended someone. That may be better than separately choosing a roofer and HVAC person. We got a pair that were use to working together and the job went very smoothly. I think the roofer charged us about $150 so it was pretty reasonable. About a week ago, we had high wind rain storm and no leaks so he did a good job.

Thanks, I can picture a "T-Top" vent. However, do they have a damper or screen. We are in a cold climate and squirels and birds are a concern. The manufacture for the range hood calls for their roof vent with damper and screen (Broan 634). I purchased one but may return it if there is a better option. Take a look at the link and tell me if you think it will last and maybe suggest a way to secure the screen in there better (it just is held into place without screw or glue, squirel could push it thru maybe. ) Here is a link that might be useful: broan roof vent

I'm trying to interpret what you wrote. The duct for your current hood is rectangular and is located 26" above the range.It is probably in the wall. Just guessing, let me know if I'm wrong. You want to switch to a wall mounted hood, which will sit higher and needs to have a round vent pipe. Right? So... chances are the wall mounted vent pipe is designed to be outside the wall rather than inside the wall. You'd likely be cutting a hole in the ceiling and feeding that pipe into the hole. Or rather, the HVAC comtractor you hire will do it for you. You'd also probably want to pull the old pipe out. There are many things to determine here, like what you are cooking with and how many CFM you'll need and whether you need make up air. You could potentially DIY some of that but I think many folks here would say call an HVAC contractor.