The only air that will flow out of the hood is that air that can enter the kitchen, so MUA is always necessary. The question is whether accidental MUA is sufficient, or is deliberate MUA needed. Generally at the 300 CFM level (which is more likely a modest 200 CFM actual flow rate), house leakage is deemed adequate for MUA unless the house is very well sealed. I you have combustion appliances, I would keep an eye on back-drafting when the hood blower is running and the house is closed up. A CO sensor may be desirable. Your municipality code enforcement officer can provide information on what regulations are being imposed.

"Your municipality code enforcement officer can provide information on what regulations are being imposed."

This is the real answer.^^^

We hear anecdotally that anything with less than 400CFM doesn't require MUA.

Kaseki & weedmeister are right. There's generally enough "accidental MUA" - i.e. the leakage from doors, windows, A/C vents, etc - to avoid any underpressure issues with a range hood as low as 400 CFM. Well, 250-300 CFM actually, after you subtract the loss from filters and ducting.

So, unless your city/county/state requires a make-up air system, don't worry about it.

Also, you may not necessarily run the hood at the highest speed all the time, anyway.