Will 400 cfm be sufficient?

Yes.

The difference between the RCS cooktop and RNB cooktop is 15 btu. That is the will all burners on full blast. That is the same as one RCS burner. Like you said having on all burners on at the same time is rare. Having them all on at full blast is rarer still.

Even at 400cfm I would still consider the RNB.

Should we go for 600 cfm?

You should definitely get the RNB if going to 600cfm.

Any brand recommendations?

Zephyr is good but I prefer Kobe.

Besty by Broan, Imperial, Cavaliere is also good.

Care to share your experience?

My setup does not apply to you but...

Ideally you want venting setup for worst case scenario.

Getting way too much cfm allows hood to be quiet during normal operation.

But if getting way too much venting power is not in the budget then venting for 95%-99% of the time is fine.

I could live with a smoke filled kitchen once a year if I can get a great Kung Pao chicken or restaurant quality steak on a weekly basis.

Thanks for this very thoughtful and thorough reply. I take your point about the RNB, but we are pushing it a bit already to go to 55k btus in this house and I think we'll be happy with that range in the end. I appreciate your thoughts on the hoods. That seems to be a whole other area worthy of a lot of research and contemplation! I'll keep these brands in mind.
Again, thank you.

To my way of thinking the important venting is greasy smoke. I wouldn't overwhelm my hood with 2 or 3 pots of boiling vegetables while I pan fry steaks if it's clear that the fumes are flowing around the hood. You should be able to time the dishes in a way to do that even if the hood can't draw out all the heat from multiple burners running high.