Unfortunately, the inches matter. The blower vents effluent *after* it rises into the hood, but will not do a lot to draw in effluent rising outside the hood. There's no substitute for a canopy overhanging (indeed, wider and deeper than) the cooking area if you want effective venting.

Note also that the actual cfms you get will be less than the nominal rating on the blowers, for a number of reasons.

Capture area matters. You need a 36" hood over a 30" to start with. Plus the fact that most people who have cabinets next to a deeper object use door limiters to not whack their doors against the cabinet, pantry, or hood. They get used to not opening the door more than 90 degrees, but no one gets used to needing to clean grease off of their cabinets.

When you are space limited you are space-limited; not much you can do about it short of a complete overhaul of your kitchen. I doubt that the slightly increased depth of the Kobe will matter much, the design is basically a flat plate with just a tiny capture volume. I am not finding the specific model you mention in the Imperial line for the sake of comparison, but since they are both under-cabinet models I doubt that you will get much more volume out of it. Could be wrong, but need to see a picture to be sure.

Anyway, the small difference in the rated blowers probably won't make that much difference overall, so what I would do is buy the one you like the best and call it a day. Consider other variables, best lighting, quietest, least expensive, prettiest design. You are plainly going to have to compromise venting performance to fit your space, but that's okay; people compromise all the time on ventilation or you wouldn't see millions of over-the-range microwaves in use in this country!

The overhang enhances capture (effluent gets into the hood aperture); high enough cfm enables containment (effluent doesn't re-emerge from the hood aperture).

kas

Thanks for the feedback. Since I am only replacing a current down draft Jenn Air range and an OTR Microwave I'm not redoing the cabinetry. Appreciate the support around having to make compromises. At least I will have a real hood. I love the looks of the Kobe, although the grease cups seem strange, but the Imperial N1930BP goes to 900 CFM's and I can add an inline blower so think I will give that a shot. Thanks so much for the input. I'm finding that replacing appliances is very challenging when not ripping out the entire kitchen! Thanks again!

Don't worry, it's also challenging when ripping out the whole kitchen.

kas

(Get whatever looks better--you're going to be so darn pleased with your improved ventilation it's not gonna really matter!)

For the benefit of anyone else checking out this older thread, let me clarify that cfm only tells you the power of the blower with nothing attached, not the actual draw with a real hood and ducts connected to it. The size of the ducting matters a lot and if your model only has 6" ducts you won't get above 600cfm of real draw regardless of what the blower can do sitting by itself. If you care about a powerful draw to capture the heat when there are lots of burners going or the oil vapor, odors etc., go for a hood with 8" ducts (the spec sheet on line will tell you their diameter) and intake filters that extend over as much of the underside of the hood as possible. Weird shapes that look as if they let the fumes escape... probably do let the fumes escape! That said, if you are not cooking on 6 burners at once or something truly smelly or oily, you may never want to turn the blower on full blast. No point buying a car that can hit 120 mph if the speed limit is 65 and the traffic is always slow. You may be paying a lot for a blower with power you'll never use.

Sometimes a ventilation hood needs to be used for ventilation, and in such cases more is better. Also fans that are run at part power are usually significantly quieter than those run at full speed for the same cfm.

I've observed enough accident reports and motor vehicle operators in action (the word driver might not apply) to know that vehicles that can barely be controlled at 80 are a lot less safe at 65 then those designed for continuous operation at 150.

kas