I had a bone stock unmodified type S head that flowed 285cfm on the intake side @ 28" of h2o.
Spent about 3-4 hours max on a very,very basic/light cleanup of the ports to remove what little casting flaws there were,and it flowed 292cfm on the intake.
Head responds dramatically to any changes...not always to your advantage though.They seem to be very touchy and can easily lose 10cfm of you work the bowl area wrong.