It will be more noisy. Not the same exactly, but I have seen cases where people fixed their bathroom exhaust fan noise by putting the recommended 4 feet of straight pipe before the first bend, instead of the less than 2 they started with.

If one has gas appliances within the heated envelope (gas water heater in a closet, furnace closet, etc) my local code requires make-up air if the CFM exceeds 15 per 100 square feet of house. e.g., 2000 square foot house would be limited to 300 CFM without makeup air.

I also think the restrictor will add noise from flow over its parts, and the fan blade tip noise will be the same. Duct noise and baffle noise will be reduced. Better would be to reduce the voltage to the fan motor so it turns more slowly.

This gets to the question of what an inspector will accept. One can build ever more elaborate means of "permanent" reduced voltage, but in the end someone can always bypass the transformer or other means used. One could also remove the restrictor, so I'm not clear why one might be acceptable and the other not.

You might be able to get Fantech to wind you a fan motor that at rated voltage pulled 300 cfm or whatever is allowed.

Oversized ducts for a given flow rate will have a low exhaust velocity and, when cold, condense grease. They will need to be insulated in an uninsulated attic.

kas

Since you are in western Canada, do consult your local building codes before deciding on the size of your blower. I had to install $7500 of heated make-up air for a 600 cfm vent, so you will likely need an even more expensive system for 900 cfm. The inspectors won't care if you never plan to use all 900, the number on the specs is what determines MUA requirements.