While I can't answer the CFM question, there are a couple of comments:
1.) You should cover ALL the openings on the topside so that the exhauster has to suck out the cut slot area only, and doesn't have to contend with massive air leaks.
2.) Some of our customers also had a hood of sorts above the cutting area with exhausting there. There is always some smoke/fumes coming off the top.
3.) Some customers used a water table; one who did thin SS had a table only about 2" deep, and his water depth was so that it just barely covered the top of the sheet. That arrangement was excellent in that it both minimized the smoke/fumes, and kept warping due to heat at a very minimum.