assuming that your exhaust ducting goes through
the roof you want to replace, I would start *there* by
putting an 8" dia roof jack in. This will allow you
to install a range hood with up to 600 cfm, now or
in the future.
You didn't mention whether your cooktop is gas or
electric. For electric, your local building codes might
allow as little as 200cfm, but I would go 300 cfm minimum.
If you have gas, or are contemplating gas in the near future,
I would recommend (and some codes require) 600 cfm minimum.