I'm not totally sure why you're worried about excess capacity. In general with any external blower, you're going to have it on some kind of a speed control. That means you can throttle it down to as low as you want, using an infinitely variable speed control like we use. From my perspective, the more flow, the better. You can always throttle it back, but you can't run a small blower at more than 100% of capacity, so I like having some reserve. But, if there really is a good reason to go with lower capacity blower, there are tons of options. Fantech, one of the most popular sources of inline and exterior blowers lists numerous options. For example, their higher end RE series external blowers run from as little as 227 cfm at 0'' static pressure (RE 6), to 753 cfm for the RE 10XL. The smaller RVF series has the RVF 8XL which is rated at 435 cfm. If you can use an inline blower, then you could use the FKD line, which is what we have. The FKD 8XL is rated at 836 cfm. That's compared to the 1266 cfm rating of the FKD 10XL we use. Here's a link: Fantech kitchen ventilation And if noise is your concern, I've posted extensively about the incredibly quiet performance of our 10XL using their LD10 silencer, so higher capacity sure does not mean higher noise levels.

If by "normal" you mean that your hood has an internal blower that blows into vents that are already in place that lead to the outside, then yes. An in-line blower is probably your easiest bet. An external blower usually requires its own housing. All this assumes that removing your existing internal blower does not alter the structural integrity or the air flow geometry of your existing hood. And, of course, the electrical requirements have to be made compatible between your existing hood and the new blower. Make sure the size of your vents are consistent with whatever your new blower specifies.

In my view, losing only 300 cfm given the duct pressure loss, the hood transition losses, and filter losses, is pretty good for a fan curve. However, I would suggest using a larger 1500 cfm fan to improve chances of successfully capturing and containing the effluent that is possible from such a large and powerful cooktop. Given the hood size that is needed, a large flow rate is needed to keep the air velocity high enough at the hood aperture. It is essential that the hood overlap the cooking surfaces by enough to capture the rising expanding effluent. Assume from each cooking surface an expansion half angle of 22.5 degrees, and determine for the intended hood height whether the expanding cone stays within the hood aperture. Don't ignore that when Broan or others assert a flow rate, they are assuming that the cooking area pressure equals the outside pressure. This would require an outdoor kitchen or near perfect make-up air system. Any house negative pressure will cause the maximum flow rate to drop per the relevant fan curve, another reason for increasing the fan capability. Broan publishes their fan curves so the effects of pressure loss can be clearly quantified. kas

I don't see why it's necessary. We installed a Zephyr Tempest II hood over our BlueStar RNB range -- 1100 cfm internal blowers -- and we've been very happy with it. This is not an under-the-cabinet model, but a 24" deep tented hood with baffles. Keep in mind that all sorts of things will bear on the efficacy of any given hood and blower unit and the amount of noise it makes. There's the installation, length/volume/#turns for the duct-work, etc., etc. Where you put the blower can make a difference, but much of the noise -- when it's noisy -- is the movement of air through the system. If it's noisy because filters or baffles rattle or even the hood itself, or the duct work is convoluted, then moving the blower will not help. I cannot comment on the unit you're considering. I will say that it's great having adequate (plus) ventilation after years without it.