The exhaust fan in my kitchen died recently.

Unfortunately, everything in our kitchen has to fit into custom cabinetry
installed by the previous owners, so that when something goes it is a
nightmare to replace. The hood over the cooktop is one of those decorative
wooden things with tiles around the edge. After extensive research, and
talking to a lot of appliance salespeople whose general response was a
shrug, I seem to have located a unit that I can have installed without
tearing out the hood or the existing ductwork, which runs up to and then
through the ceiling to the outside.

Now, I do a lot of cooking that produces heavy-duty steam, smoke, and smell.
(The cooktop has a grill in the middle, I do a lot of things in a wok, I
cook Indian food, I make giant pots of stock that simmer for a day, and so
forth.) The old unit--a Thermador squirrel cage thing, circa 1980--was
grossly inadequate, and led to the accumulation of greasy dust on various
surfaces in the kitchen. In addition, the ceiling in my kitchen is high, and
the kitchen is not closed off from the rest of the house, which is a big old
house with high ceilings. My dream fan was always one of things that is
mounted outside the house, but I discovered that it would require larger
ductwork, which would be a big job involving going into the ceiling, so
that's out.

The unit I've found moves about 400+ cubic feet per minute. This seems to be
the most powerful unit that can use 3 1/2x10 ductwork, which has a capacity
equal to 6" round, according to the appliance people. I'm wondering if
anyone here can offer an opinion, based on their experience, as to whether
this fan is going to be adequate to my needs. (It is over a 36" electric
cooktop--unfortunately, I would SO love to have gas--so BTUs are not a big
concern.)